The oldest machine I'd be willing to back-grade to would be a 580 SL. This is gonna be long, but I'll tell you why...
I'm 39 yrs old, my dad started my company the year I was born. We started with 580 B's, I'm not sure how long the B had been out in 1972, but I do know that some of them we had didn't come with a ROPS canopy on them. Our mechanic installed the factory-supplied ROPS in the field on at least 2 of them I know of. We had a total of five 580 B's, the oldest being '72 models and I think the newest being about a 1975 model. The '75 model had a power shuttle, the other 4 were clutch machines. These are the machines I first learned to operate on. In 1977 I'm pretty sure (late '77) we bought 3 brand new 580 C's. 2 wheel drive, standard hoe, cab with heater. These 3 machines were the biggest pile of $h1t we ever owned. All 3 of them... Had Case work on them numerous times, our own mechanic worked on them... They just never were right. The only way you could lift the back of the machine off the ground with the back bucket was to raise the front bucket. Didn't have enough power to overcome the leverage of the extra length of the machine. if you got in mud bad enough to have to walk it out with both the front and back end, you might as well hook a dozer to it. They didn't have enough hydraulic power to work both functions at the same time. Cab was horrible on them, you couldn't tell where your back tires were when sitting on the seat, Real fun to load or unload off a trailer when you can't tell where your tires are. And the cab was an oven in the summertime.
One of the C's got stolen in about 1980, and we traded the other 2 C's in on 2 brand new 580 D's in 1991. Keep in mind, we traded the C's in, but still had 5- 580 B's... The operators preferred the B's over the C's... The 580 D's were both 2 wheel drive, standard hoe, open ROPS. The 580 D's were good machines. We kept both of them until 1990. In June of 1984, we bought our first 580 Super E's. Bought 2 of them at the same time. One was stolen 2 weeks later, so we bought another one, then in 1986 another one was stolen, so we got another new one. I beleive we traded the 2 oldest B's on the first E's, but not sure on that. I know from about 1980 till 1995 we had 7 backhoes at all times. Now about the E's, for their time, they were great machines. You could tell a lot of difference in power with the E's compared to the older machines, and hydraulic speed was quicker.
Now, why I wouldn't go back to any of these machines... These had all "evolved" from a farm tractor type base, starting I think with the 530, then going to the 580 Construction King, then to the 580 B. They started with a basic tractor, then added a loader and a backhoe attachment. State of the art for the time, but that time was 40 years ago. I can remember the whole backhoe attachment becoming loose when a Tie-bolt would loosen or even worse break. They had the steel tube backhoe boom, which just about every one of the machines mentioned ended up having to be welded back together at some point. Twin boom lift cylinders, that greatly reduced your view in the trench with a small bucket. We re-bushed MANY of the swing towers and bottoms of booms on these machines, they'd get so much slop in them that it was horrible trying to dig with them. And we were constantly re-packing hydraulic cylinders it seemed like. We replaced several bucket cylinders on the D's, as they'd get bent. This was fixed on the E, as they went to a beefier cylinder.
In 1990, we bought our first 580K. Again, it seemed to be a step backwards. The old E's we had would out-dig it. And the backhoe control levers had a habit of sticking. Also, the 4 speed lever for the transmission came up out of the floor on the right side, and by the time it was 6 months old, if you didn't know the "technique" for shifting it, it would bind up. Brakes were kinda crappy on it too. In 1991, got our first 580 K extendahoe. It was the next "Phase" than the first K, and was much improved. Sticking control issue was resolved, shifter was moved into right had console, and it actually had power and speed. In 1993 I bought a new 580 Super K. Again open ROPS, standard hoe, 2 wheel drive. It was another "phase" up, and was a very good machine. I kept it until it got stolen in September of 2009. Never had a major repair of any kind on this machine. It did have the common electrical "gremlins" the K's are famous for. But otherwise was a very good machine. In 1995, I bought my first cab machine since the C's. Brand new 580 Super L, cab with heat and air, standard hoe, 2 wheel drive. I wanted a 4x4 extendahoe, but it was going to take a couple of months to get one in, so I bought this one. It didn't have the A/c installed when I bought it, but a factory air unit was figured into the price, and installed when it came in about 2 weeks later.
Now here is some advice I'll give everyone... I can't imagine a used machine with air costing a lot more than one without A/c. Even if it doesn't work, it can be fixed. I WOULD NOT BUY A CAB BACKHOE WITHOUT A/C UNLESS I WAS ONLY GOING TO RUN IT IN ALASKA OR ANTARTICA... I bought this backhoe in April of 1995. I'm in Oklahoma. I ran it a week, then parked it for the other week until my air was installed. IT WAS LIKE SITTING IN A GREENHOUSE. Well, a greenhouse with a motor on the other side of the glass, and hot hydraulic components in the floor. I'm also kinda fanatical about if I have a cab machine, I don't want the interior of it to be covered in dust and dirt. I do dig most of the time with the back window open, but never open the side windows. I know it's bought to work, but when I spend over $50,000 on something, I kind of like it to be kept clean. Like people that will open all the windows because they're too lazy to clean them... One of my pet peeves.
Anyway... I kept the 580 Super L until September of 2009, when I used the insurance money from the 580 Super K and the Super L as trade in on my 580 Super M. The Super M is Cab with heat and air, 4x4, extendahoe, ride control, pilot controls. I love the pilot controls, as I actually run an excavator most of the time, so the pilot controls are natural for me. The 4x4 is worth it's weight in gold, as is the extendahoe. It's not hard to get used to the extra reach, but is hard to get used to not having it when you get on a machine without it. The ride control is great, not only for roading, but just on a rough jobsite it makes a world of difference. I've got about 1500 hours on the Super M now, and (knock on wood) have had absolutely no problems out of it. I never had it head to head with my Super L, but I think it'll out dig it, and that's saying a lot to me.
Now, we did have a 580 M, Open ROPS, 4x4 extendahoe that we bought in 2006 I think. It came off of the Case rental fleet, so I don't know if it was turned down or anything, but I was never impressed with it. The 580 Super K would out dig and out lift it, and the Super L and Super M would blow it away. You could park it side by side with any of them, and tell it was a lighter machine. I think they made a lighter version of the 580 when they did away with the 480, and just made it the "plain" letter instead of a super. We actually got so much mud packed up in the rear end of it once, that we had to pull the back tires off, and clean the mud out because it wouldn't pull itself. Pretty sad when it wouldn't go, but the 2 wheel drive Super K just kept wallering through the same mud, bringing rock to my gravel box. It also seemed like the straight M got a lot of slack in the pins and bushings on the back end awful soon. Even greasing it everyday.
Well, Seems I wrote a book, but I think I pretty well covered everything I can think of. We've never replaced a motor in any of them over the years, I don't remember any transmission problems, we did shell out a rear end in one of the D's and also an E, but no telling what was done to cause that. That was back in the "pipeline" days, and these machines took a LOT of punishment. As of yet, havn't cracked or broken one of the ductile Iron booms, and we dig a LOT of rock. I'll use an excavator whenever possible, but we work a lot of places you just can't get a PC200 or larger into. I've laid 8" ductile Iron sewer and set precast manholes with my Super M. I don't recommend it, but it can be done...