14 series would be my choice of machine. Lots of aftermarket parts out there. Straight cut header probably would be the 810. Try to find the large auger if you can they do feed better. Things to look for would be worn auger flighting on the clean grain lower auger and the one up in the tank. The 4 or 5 augers depending on size of machine under the rotor will wear at the front where they run in the wood blocks. Not to bad to repair if you have a lathe. Elevator chains should be checked and the sockets that run them. Pull the cover on the horizontal unloading auger and look at the flighting. Most of the 90 degree gear boxes have a grease zerk that you should grease while in there. Grab the belt that comes out of the pto housing on left side of motor and pull it back and forth. That will give you an idea how much play is in the splines on the hydro shaft. If you see oil under the engine it probably is the pto shaft seal that is starting to leak. That can run upwards of 2k to repair. On the clean grain elevator about half way up look where the chain passes by that drives the return. They tend to slap the side of the elevator and will wear a hole in the housing. The rotor probably will be the standard rotor in a 14xx but could have been updated. Drop the concave's all the way down and you should be able to see the rasp bars pretty well, kick the rotor in neutral so you can turn it. Also the rotor should stop and not find a heavy spot if it is in balance. The shift lever is under the radiator. Check the 3 pins and bushings on the rotor belt variable speed pulley to see how much wear they have. Pull the front cover for the rotor. Then you can check the elephant ears. While in there take a flashlight and check for holes in the cone, also the bolts where the cone bolts to the cage wear the heads off. I would just plan on replacing them. Check the vains in the cone and cage for wear. Most wear will be on the left side of the cone. Check the floor of the grain tank for holes and also check the grain sump. They came both with and with out straw choppers so be sure to look for that if you have a preference. It is inside the machine right behind the rotor. Check the gears on the auger bed augers and the spinners at the back of the machine. They do wear. They also run with a lot of  wear on the teeth so some wear is not that big a deal. The cleaning fan on many of those old machines have been up graded to the newer style. What I have found is you need to run about 200 rpm faster than book settings with the cross flow fan to get it to do a good job cleaning. Feeder house check the floor for holes, chain and sprockets for wear. Mine cracked on the left side up where it pivots. The finals maybe someone else can tell you what to look for. I run a 1470 hillside so finals are different. In fact the whole bottom frame is different so I can not tell you where to look for cracks etc. In this part of the country most machines came without a heater. So I would check to see if its there if you need it. The heater knob is a water valve in the left rear corner of the cab above the gear shift lever. Early machines will be cable operated hydraulics the later machines will be electric over hydraulic.  

A lot of this would also apply the the JD when it comes to wear on sprockets and flighting. Best if you can grab someone who knows these machines when you find one or two that your interested in to help you look. Hopefully this will help some if you have to go it alone. 10K around here would find you a pretty decent machine. Good luck in your search.