Apart from the usual things to check when buying a car here are some 205 GTi specific things to check;

The floor pan in the boot rusts away as this area wasn't factory undersealed.
The arches tend to rot away under the plastic wheel arch trims due to dirt accumulating there.
There is a steel coolant pipe bolted to the inner O/S wing down near the crank pulley that is crimped into a rectangular section that rots away too.
There shouldn't be any knocks from the suspension, check the rear tyres for scrubbing as this most probably is from a bent rear beam (they can be straightened but it's easier to buy a 2nd hand beam).
A sloppy gear lever is probably from one of the linkage ball joints ovalling the hole through which it passes in the front subframe (either get the hole drilled larger and fit a top hat spacer or take the frame off and get it welded).
Rear beam mounting bushes split cause the beam to move (check this by driving slowly forwards and backwards past an observer and yank the hand brake on and see if the rear wheel moves backwards or forwards relative to the arch).