Welcome to the site, Im new too, the advice i would give when looking for a disco is: Go and see as many as you can!
Check for crunching in gears between 2nd and 3rd and visa versa!
Check behind the cylinder near the bulkhead for leaks from the head gasket on tdi's.
Check inner sills, boot floor and inner arch/wings for rust!
Sunroofs tend to leak so if you find one without, Game on!
Electric windows and central locking, esp back door tend not to work! Not hard to fix but a good bargining tool!!!
Hope this helps!