Make sure it has the duel stage clutch. I personally would not want one with Multipower. I know some love it, but I don't need it and it may be the weakest link in the transmission if it has it.
I am currently working on two 35s (one gas, one Perkins diesel) and have never had a problem getting parts. Just about all parts where bought through various online sources. My experience with dealers (both Fords and MFs) has not been great. Either they don't know much/don't care about these 50-60 year old machines, or charge crazy prices (like $24 for the fuel inlet screen for the carb when TSC has it on the shelf for $4). You don't really need power steering on a 35 as they are not a heavy tractor, unless you want a loader on it, however just about every part is available to rebuild the power steering box. Not a cheap project though, so if you are looking at a 35 with power steering make sure it works. Inspect the hydraulic top cover under the seat and make sure there are no cracks caused by abuse or incorrectly adjusted hydraulics.