The good news is it's not super old, Gehl is still a good brand, and the price is super low. Are you sure the seller is not a scam? The best brands for a mini excavator used are likely Kubota, Takeuchi, Deere, and Cat. Maybe also Hitachi, Kobelco, and Komatsu. At 18K, is the engine smoking, hard to start etc?
Most good used machines run in the $40K-65K range with 2000-3500 hours. Parts are hard to get from Gehl, but it depends on which ones. If this is a machine you are going keep a long time, spend more money and get a better name brand. If you just need a machine for a couple of years and likely for just a few hundred hours, you might be able to get by with a bit loose, and leaky machine etc. Make sure the seller is not a scam, and don't transfer any money until you've seen it in person etc.