DO IT!!!
I'd had my series 2 rst for 7 years and bought a mk1 frs last summer and absolutely love it!!
Although there are things i miss about my rst (was 260 odd bhp so quicker than my focus at the moment), the old school feel and presence and as it was a track car doing track days, the focus is far better in every other way and it hasn't missed a beat in 6k.
I paid just over 10k for mine with 40k on the clock and all the basic mods done, which i'd recommend if you're going to mod anyway as the basic parts are quite expensive (exhaust/decat/air filter/map/manifold etc).
Plenty to choose from so go and have a look at a few to get an idea of what ou can get for the money.......there are some around for 7k odd but they're unlikely to be minters