Howdy and welcome to Airforums.com, Atraveller! You've come to the right spot for questions and advice on buying yourself a nice TT (travel trailer).
If you want to really get an idea of what condition that '74 is, go to
Janet's checklist and print it all out. Go back to the trailer and go through it line by line with the seller there helping you with the answers.
Then come back here, also with a ton of photos, and we can better help you to sort out what kind of a deal you are looking at.
Some trailers from that era could still be in pretty good shape, but there are also lots of nightmares out there, too.
If the frame is solid and there are no holes in the floors, there's a good chance you can get it back on the road again without spending too much. I am assuming here that the last five year's worth of rentals was just stationary, not travelling.
You can count on needing a new fridge (refers we call 'em

) a new water heater, and very likely a new furnace. Maybe not all right away, but if the originals are in there, they are at the end of their useful service life. Count on about $3,000 for that, including a new refer, furnace, and water heater.
So as you can see, the initial investment is really only the beginning, unless you are dealing with a TT that has really been looked after all those years. By the sound of your description of its life, that doesn't appear to be the case, so that's why it's important to detail ALL that needs done BEFORE you spend that first dollar.
There you go, the first job is some investigation work. We're looking forward to hear how you make out!