If you can get the S/N, look it up on www.vintage-trek.com, and see exactly what it is, at least based on available history. Trek did a lot of mixing and matching of tubesets, tube makers, and geometries back in those days. I suspect it's not all 531, but you should get a real good ride from other materials. The x10 geometries are usually sport-tour - sort of racing and sort of touring. Most people love them. All the threadings and hole sizes will be standard English/American - you should have no trouble doing upgrades or parts replacements.

If it's in real good riding condition with no significant corrosion, I'd say $250 is not bad. All original is good - you can use the vintage trek site to verify if it's all original. Some Treks were commonly modified by the selling shop or built at the shop - I'd think a 510 (upper middle of the range) was usually delivered stock. I have a 1984 610, and it's definitely a good bike. Not perfect, but good.

Road Fan