I just went through about same thing last week when I picked up my 90. He was asking 1500 said it was in great shape, rebuilt carb, new front sprocket (not installed) blah blah blah. I get there and although mine wasn't filthy. The chain and sprockets were toast!! No battery, would not run without choke. Oil way over serviced, petcock leaking. Thought to myself this is excellent condition!! So he was asking same 1500, offered 1000. and he took it. That day got on the net and got everything coming. Chain and sprockets done, oil/filter changed, actually engine looks great inside. Swapped carb out for $24 chinese/ebay carb. New plug, air filter. All I have left is battery and petcock, which are in route. It runs great down the road now. Just have the nasty fuel leak at petcock and dim lights
I too had my doubts like you, but these things have been made for years and they are common yamaha, so parts are easy and cheap to find. If you can tinker, I think you will be happy in the end. I can tell you putting around town, everyone looks at it wondering WTF is that. Even had a guy ask me if he could take a picture of it LOL.
Here is a link to a parts fiche for you bike, comes in handy! and it lists 5VC-11351-01-00 which replaced a 4CS-11351-00-00 which they list at only 6.27, gotta love dealer markup