Main thing to do is buy one with a good history, build thread (so you can see the stages of it's work and progression) and never buy on condition over mileage, a higher mile car well maintained will be far better than a neglected low mile (or apparent low mile car).
Ideally buy from a forum member and remember to look at things like arches (rolled or not etc), I personally didn't want one with rolled arches and I have kept them original and rust free.
Under seal is fine and expected on one but be sure to just check everything you can and get a feel for the gearbox. Signs of issues getting into gear at high rev's is a sign of the clutch going, for example when my clutch went it was fine at low rev's up until about 7K then above this it was struggling going from 3rd to 4th.
There are reasonable ones within your budget but I would personally look at ones that are on here and have a history and build on here you can follow and see how the car's been looked after.
Where are you based as it maybe worth looking around a very good one before hand if you can just to get an idea of what you'd be looking for.