Look at your brakes now. If you have 4mm to move the pads down, you good for 700. As far as compatibility goes, if you're changing the seat post, stem etc you're doing way too much. Just look up the brand/model when the craigslist ad shows up and there will probably be several threads in the vintage section talking about how standard the thing is. If the name sounds japanese and it doesn't have cottered cranks then it is probably a winner, but double check anyways.

If you end up dropping any real amount of money into this project, then buying a new frame and components will be way less of a headache but can still be well unique.