The best advice i can give is look around the FS section on here and carry out a post search and you will find out what has gone wrong with the car since that owner has had it, it will then give you an idea on the type of car it is i.e. shagged or not shagged lol
You will find that there will be R's for sale at both ends of the scale, some for peanuts and some for big money. Depending on what your requirements are then i would go for a middle ground R as the cheap R's are probably on the verge of breaking and will ultimately cost you more to bring it back to a standard you want, or go for a big money R where the majority of mods are done for you, me personally i dont see the fun in that one however it is ultimately cheaper as the mods aren't cheap.
Go for a middle ground R and get a few sensible mods e.g. exhaust filter etc but also keep the reliability of the R to a higher level than a highly modded R.
Range of prices i would say 2k to 9k (ive seen this) so you can see there is a big jump.
This is the opinion of myself and probably isn't the same to alot of others however it all depends on your individual needs and wants.
When you take a look at an R ideally take someone with you who has experienced an R and can point out the common faults and quirky ones at that lol.
With all this dont let it put you off mate. Good luck in your search and welcome to the OC;-)
Ant.