H. I think the RS is a fantastic car, definitely the right choice for me - I don't use it that often and it is a 2nd car which is important. I use it only for fun miles which is when it it is at its best and keeps the mileage down a bit (I have ltd mileage insurance policy) but I plan to go to the continent a couple of times this year where the roads are more enjoyable. If you were using the car daily, I think you would enjoy a regular C2 more but when you go for a blast or a track day the RS is in a different league. Drive the cars and see what you think, its the only way to tell.
Regarding where to buy and what to pay, I think you have to spend the time and familiarise yourself with what is on the market and go and look at some cars. Once I had decided on an RS (I was looking for a LHD 993 or 964 RS) it was a little easier as they are quite rare and I probably new about every car that was for sale or about to come up for sale. There are also a couple of very helpful dealers who specialise in RSs who were very honest and a great source of information.
964/993 C2s are a bit harder as there are a lot more for sale. There are some nice cars available through the Porsche Post or this forum but you need to take a look at some. Also hard to comment on price, depends whether you want good useable or really exceptional as the price will vary considerably. Again, look at some cars so you know what is out there and when a nice car comes up at the right price, you will know.
HPI check and an inspection is always a good idea. While I think a good service history is important, pay particular attention to what has been spent in the last 2-3 years as if the money hasn't been spent in that time you will be spending it soon. As for the C2S, if you like the look then shop around. I think there are quite a few of these around at the moment as people are asking big money for them and that makes them harder to sell. I went to look at one at a London dealer and he was asking 40k - stunning car but too pristine and expensive for my brother. The guy then called back several times and kept lowering the price until it wasn't too far off your budget. I wouldn't discount the car, it looks great and has all the other C2 virtues.
Good luck with your search.