I can't think of anything specific to the R5 (interior trim is maybe a bit flimsy), but I've got a few suggestions which might be useful if you've never bought a car before.
This might sound like stating the obvious, but it's worth checking that the chassis number, registration number, etc. on the registration document match the ones on the car. Is the seller's name and address the same as it says on the paperwork? How many previous owners has it had? An HPI check will tell you if the car's stolen, not paid for, been written-off, etc.
Keep an eye out for signs of recent body repairs too (paint overspray on trim or under the wheelarches, etc.) as it might mean the car's been in an accident, which if it's not been repaired properly could mean trouble. Alternatively it could have been done to hide rust. Look at every panel, one at a time - does the colour match, are the gaps even, etc? Look at the tyre tread - the tyres should wear evenly across their width - if they're worn unevenly that could mean trouble too.
Look at things like the steering wheel rim and pedal rubbers: if the steering wheel rim is smooth and shiny, or the rubbers worn the car will have done lots of miles - not a problem if it's been properly looked after, but beware if the mileage on the speedo isn't that high....
Ask how often the car's been serviced, if anything major has been done to it recently, and ask to see receipts. Ask to see the MOT inspector's report (the form you get with the certificate), as it may mention forthcoming nasties (it'll say 'advise' rather than 'fail'). It may have been 'lost' of course....
Make sure you have a drive - does it go in a straight line with your hands off the wheel, do the brakes work (without pulling to one side), do the gears change without crunching (especially when changing down to second or third), does the exhaust smoke much?
Take everything the seller says with a pinch of salt, and if you don't like the look of him / her don't buy the car! It's worth taking a friend with you (preferably someone who's bought a car before), and if you're not sure about the car don't buy it - there are plenty of others out there.
Not sure about the price as I'm in France (prices are much higher here than in the UK) - have a look at finished auctions on eBay to see what other R5s have gone for. It's always worth haggling anyway.
Good luck!