As to bearings, the majority are sleeve type, not a huge deal to rebuild if needed.
Chains are mostly available just need to know where to look.
Belts are not a huge deal either for the most part. Belts on the machine will all have aligator splices and if you know someone who has a round baler that replaces belts as part of regular maintence just ask for their castoffs. Otherwise just a matter of opening up the checkbook. The main drive belt is a bit of a different story as splices are not real acceptable here.
Grainpans (under the cylinder) will commonly rust out but can be patched.
If it has Rockwood paper pulleys it is just a matter of opening up the checkbook again. PaperPulleys in my experience has very rapid turn times but range from about $50 to $200 per pulley.
Really the key thing is condition when you start. If the separator has set in the grove when the owner got a combine in the 50's my thoughts are consider it a parts machine. If they put it in out of major weather it is likely worthy of being made a runner.
Not a expert here but these are my observations having worked around this stuff some and I did own a 22" McD machine for about 9 months.