Don't know how much for the mk3 but the mk1 hoods are usually between 200 - 300 to buy and anything from 100 - 300 to fit professionally. Do a search in the mk1 section for "roof" or "hood" and there should be some links to websites that will supply mk3 roofs.
As for what to look for, not sure on a mk3 but get her a mk1 convertible, they look so much better.

There may be some info the mk3 section on what to look for though, so do a search in there. If you are wanting a mk3 then there's user RandomJord on here selling one (or he was the other week), so have a look in the for sale section and he's just done loads to it and got bored of it so selling it at a very reasonable price too.
I don't even know if it's still for sale, but Jords mk3 cab is here
http://www.golfgtiforum.co.uk/index.php?topic=57535.0 I've even done the hard work for you.