Mk1 RS owner speaking here....
Bought mine 5 months ago, private sale, 40k on the clock and got it for a great price.
It is modified with Dreamscience, K&N Gen2, Piper Exhaust system (de-cat), Forge Re-Circ valve, Uprated Charge cooler radiator and roose hoses. Running 276bhp and its an animal to drive
Mine is used everyday and is almost as practical as a regular Mk1. The ride is quite firm with the racing sachs shocks and 18" OZ wheels. The LSD does a great job of keeping grip on the front wheels and only gets a bit twitchy when booting it down bumpy country roads.
The 330mm Brembo 4 Pots do a great job of reigning in the power and adding braided lines makes for pin sharp braking.
Running costs are not as bad as people think, infact I would say it would probably costs just as much if not more to run an ST (Fuel, tax and insurance).
£30 of Shell V-Power can last me 5 days of normal driving (keeping the boost down). Of course when thrashing it the fuel guage just drops and you will soon find your self filling up again. Insurance is group 16A and relatively cheap compared to other performance cars.
In terms of reliability the Focus RS is almost bomb proof with very little going wrong with them apart from a few common faults.
For me it has to be the RS just for the smile it puts on my face each time I drive it and to hell with running costs.
If your looking at buying a performace car, running costs should be right at the bottom of your list.
Will get some pics up soon...