Does anybody know if on my fiesta st, I can keep my original exhaust and just buy a decaf for stage 2 or do I need a new system overall? Thanks.

Just a word of warning, decatting your car will fail an M.o.T. Mine was decatted and passed emissions tests (So you can get it passed at places like Halfords), but just letting you know. I bought an ST-2 that turned out to have a stage2 remap, without the intake or exhaust (But left the decat, thanks for that previous owner, you pleb). And because the decat was a different size to the stock exhaust, my car would resonate at red lights, sounding like it was constantly slightly revving? So from that experience, I'd recommend either getting the overall system, or just making sure that the decat isn't bigger than the stock. 

You'd probably need to map out the O2 sensor as you might get an EML light come on when decatting. The car might run a little rich if you leave the decat on with the engine light.