I am hardly an expert but in terms of ford power
Duratec > Zetec > Crossflow
As far the revs I believe the big hp Duratecs rev to 8500 which is the same at the later S2Ks. The S2K may have (slightly?) less low end torque than the Cosworths but is cheaper and will give 240 bhp all day everyday.
If $$$ is no object its an easy decision to get a Caterham R500 which will have better sales support and relatively easy parts availablity. If $$$ is limited you need to decide between lower power spec reliability (eg Cat R300) or a more powerful but less supported car (eg: Ultralite, Duratech Birkin). It comes down to the old racing: Cheap, fast, reliable - pick any two