Not at all. Yams are built like the preverbial #*!t brick house. Parts are plentiful, as its old technology. Having said that marine parts are never cheap, especially for gaskets, thermostats, etc.
I sold a Yamaha 25hp, same age for £500, palletised a few years ago. I'll bet it's still running. That engine was sweet as a nut, and was easy to maintain.
Don't just hand over your hard earned cash though. See it start from cold, measure pressure across both cylinders, etc. Renew starter rope every other season. The 25hp needs a good tug to get it going and I've seen the rope fail on poorly maintained engines.
I'd also be asking for receipts, and history.
There's a Yamaha 2003 long shaft 30hp electric start at Seamark Nunn for £700, so an '86 engine should probably be nearer £400 tops, unless it's mint.
Is that with or without VAT?