I am currently restoring a GT14. I got mine very cheap, but cheap for a good reason... it was beat to death. I have found parts availability very very good. Most stuff is still available, and not really too expensive, with a few exceptions.
Problem areas appear to be the front PTO system, there were several late changes to it that only dealers know about. Gary Jamieson in PA has been very knowledgeable about that part. I'm a mechanical engineer, and started to rebuild the pto on mine, and got my butt kicked on it. If you go to buy a GT14 / YM146, make sure the PTO is in good shape, or factor some cost into your price for a rebuild. Parts for it are NOT cheap. Wear items appear to be priced higher than non-wear items. Small brackets and bolts, etc are cheap, while wear items like mower wheels are expensive. From what I've seen, the GT 14 is a well designed tractor, with a lot of nice features, except for the PTO. You get a real water pump, hydrostatic drive, electric pto, etc. Prices seem to run about $2500-3000 in decent shape. Perhaps a little more if you have the 3pt hitch or rear pto which were optional. Buy the manuals. Expensive but worth it. I can't imagine maintaining one of these without it or a knowledgable dealer nearby. I don't have a dealer near, so I had to buy manuals. Good luck.