What to look for... and also what not be concerned about...
Rattly variator (diesel idle sound) common on these earlier Twinsparks is quite easy to fix - easy to replace the variator while the cambelt is being changed - can be changed while leaving the camshaft in place. At first the sound is only there when first started, but eventually becomes continuous at idle.
Sticky/smudged interior plastics are typical, can be repainted. The bolster at the side of the driver's seat base is relatively easy to repair, so that's also not important. Interiors in general wear pretty well. Spider sound system/stereo usually worthless/useless so any upgrades are worthwhile if done caringly.
Dents in doors and rear side panels are typical, most can be removed by dent magicians. Mine was able to drill a hole to fix a rear-quarter dent. The fit of the front and rear bumpers (gaps, angles) is usually quite poor, so problems there might not indicate past damage.
Mismatched paint colours (especially red) are common on rear hood cover. If car is red, check the match between panels very carefully. Basically, only buy a red car that's either completely original or completely resprayed. It rarely matches otherwise.
Do wheels need refurbishing to fix gouges and dull paint?
Check clutch is not too heavy and not making a creaking noise - if so, then probably needs replacing. Not a nice job.
Check gearbox engages all gears easily - these are usually quite a slick gearchange so a problem here would be unusual and worth avoiding/budgeting for replacement gearbox. My '97 Spider had teeth missing from reverse gear - horrid thumping noise in reverse. I did repair it but quite a long job.
Check condition of hood - missing stitching? Rear window slack and wrinkly?
Assess condition of engine - cleanliness of oil, cleanliness of coolant. Look for stray/tampered-with wiring around the engine bay. Try to buy a car that looks like it's enjoyed caring maintenance.
If it all seems original and is good in most of the areas mentioned, then it's probably better than average. These cars seem very cheap in England so you might as well find one of the better ones.
EDIT: the Buyer's Guide that Philbass39 linked to is excellent, has most of the points I mentioned plus a few more. Note the useful breakdown of costs - but also note that time has moved on since 2004, so there are some more age-related deteriorations to expect.