I would have to agree with most of the other posts. As I read your post the first thing that came to mind was the question (why have they been replaced so soon?). I am not saying to stay away from it if you think it's a good deal. Just be sure to do your due diligence. I would recommend that you contact the local Deere dealer to see if they serviced this machine or at least if they have the oil sample history. You will need the serial number to do this. Oil samples are MUCH more valuable if you are doing them over the life of the machine. Each machine is different and the higher number of samples will help determine compartment wear and add to the equation of how to predict remaining compartment life. A single sample can only help determine if catastrauphic failure is iminent. Samples must be taken after the compartment (and oil) has reached operating temprature. Information on the compartment oil should be given as well (when turning in the sample). Information should include brand; viscosity; age (hours of use), type; and wether or not oil had to be added to the compartment since it was last changed (to help determine if and how the oil is being consumed or is leaking). Without this information the information from the test is less accurate and paints a picture that is less clear. The dealer should also be able to tell (if it was serviced at the dealer) what & when has been serviced/repaired there.
The undercarriage should not be worn out yet (my 315 shows no visible signs of wear @ 4,000 hrs), but that can be explained by having done LOTS of traveling (or walking) or that it was driven over rocks for most of it's life. Either way if it's been replaced with a good quality undercarriage you should see it live for 10,000 hrs+ (IMO). Make sure that it was replaced completetly instead of....strike that. There is to much stuff to look for (for me to explain here). Have the product support rep (from the dealer) come out and evaluate the undercarriage. They can tell you what is good and what is not and where each component is in relation to it's useable life.
If the UC checks out the pump still could be a deal breaker. If the pump failed this early it is probable that it had a catastrauphic failure that may have poluted the entire system with metals. When this happens it can be difficult to get them out. Even if you do many times the metals may get stuck in seals and packing. The "floaties" may be gone but the ones stuck to things will create excellerated wear to the places that they come into contact with. Excessive "drift" may be evidence of bad seals which may be the result of the scenario I just described. The sample on this compartment is a must. Removing and cutting filters can help in the investigation also.
If your gut is telling you that the machine may be right for you than the investigating is warranted. This machine may be fine, just be careful. Do your homework. Most everything you need for your investigation can be obtained at the dealer. You will have to pay for some of these services. If you are serious it will be $ well spent. Good luck.