Stage 2 prerequisites are typically FMIC, 3" DP with a sports CAT, air filter of some description (which you have)... optional are things such as Forge DV (which you have), silicon hoses (mainly a TIP) and an aftermarket exhaust but most important things are the bits that aid air flow and help keep AIT's and EGT's low so that you can run a bit more boost and timing which is pretty much what a stage 2 is...
Cost wise as you already have a Revo map I think its £50 to go stage 2 but obviously the other bits add to the overall cost... the thing to remember though is that a stage 2 doesn't give you much more power over a stage 1 but in conjunction with the upgrades you will have fitted you will have most of the power most of the time rather than all of the power once then some of it for the rest... heat is an engines nemesis and anything that keeps AIT's and EGT's low will give you more sustained power..
<tuffty/>