As Robert Cross said, axle replacement is probably in the very near future for this trailer. Also consider the fridge, A/C, hot water heater, converter, and other smaller things as possible replacement very quickly. After 40 years, any and all of these items are likely to quit working at any time. If you are able to do the work yourself the cost is not quite as large. Factor in the costs of doing the above when you consider purchase. You could easily spend another 10 grand in repairs. There is an Inspection form available on this site. Take it with you when you look at the trailer. Good Luck!!
2012 GMC 2500 HD Duramax Denali
1976 31' Gone but fondly remembered
1976 22' Argosy