i'd have no problem with it. Old tanks were generally built to go to 200psi and were much thicker than todays stuff. If it was drained regularly its quite probably still very healthy. You can actually open caps on the older ones and look inside with a light.
A lot of people wouldnt be comfortable with it, if you're in that catagory you can either purchase it for the pump and motor, or you can pass on it and wait. Around my area newer [10-20 year old] large air compressors like that, especially 3 phase, are plentiful for the $500 price range and quite often less simply because people just want the space that these behemoths take up back.
In that compressors credit it sure is externally very clean and corrosion free for 55 years old.
As far as parts you shouldnt have a problem, i believe champion is still in business and believe it or not but the pumps often dont change much. Despite all the outsourcing that goes on today most of these companies do maintain a parts stock for old machines simply because they are so reliable and people do rebuild them.