Try to get a BA Mk2 if you can rather than the MK1. Although you may be limited by your price range, try spending that little extra in getting a 'newer' car with more options etc, rather than the basic 'taxi' options. Avoid the 5spd manual if your going to even slightly modify the car, unless of course you can afford a few $k on strengthening/better clutch. Find a car with at least a few months of warranty on it, so if you do happen to notice something major after purchasing, you 'may' be able to get it fixed under warranty. And generally be careful with buying a modified car, make sure that the owner knows a thing or two about cars and didn't just get the usual mods done without spending the money on strengthening the engine etc. Its all well and good to turn off emissions and up the boost with a 'tune', but the engine/driveline was engineered to have those restrictions in place. If you can hold off until later this year or better yet until Orion is released in march next year, then you will no doubt be able to pick up a bargain around then. Consequently if you do happen to buy one now, don't be surprised when your resale goes through the floor in 6-12 months time. The most important thing when looking I guess is seeing who the previous owner was and what type of servicing and repairs were undertaken. Someone who looks after their car would have had most issues fixed/replaced by the time they are looking to sell/upgrade. Whereas someone else might be selling their shopping trolley because one of the wheels are starting to wobble.
