To give you some more constructive information;

More important to look for receipts for work done, service history etc. A recent setup from a reputable tuner (check on here) is a good idea.

As most cars around your budget will have high milage I would avoid most cars that have been modified unless done properly. One tip is fit a turbo oil feed with a filter in it. Autospecialists do one for £50. MSD Magnetic sump plug is worthwhile too £8 or so.

RSTs need frequent oil changes using high quality fully synthetic oil. Generally if it smokes blue, white or black out the exhaust whether on boost or overrun then its a sign of future problems.

If you do want to modify then best getting one with the things done that you want already fitted using good quality parts as it will save you £££ in the long run.

Check the whole car thoroughly for rust, in fact worth paying the AA to check for you if you don't know what you are doing. Make sure the VIN on the front panel matches the VIN under the plastic panel on the floor next to the driver's seat as you open the door, otherwise it could have been reshelled or a ringer.

Make sure the car starts from cold and idles around 1krpm when warm. The fans sould kick in automatically also once hot otherwise theres something wrong. Turn it off and make sure it starts when hot as well.

90 spec ones are more sought after on a G or H reg but most folk tend to fit various parts from the 90 spec cars such as interior, bumpers and rear spoiler. H reg ones generally have better bodywork because Ford changed the manufacturing location and used better quality steel at the time.

One more thing don't drive it in the snow or during winter as the salt on the roads will eat through the chassis before your next MOT is due.