The full kit, inc a standalone ECU for the S2000 is just under £2000.
They give 20bhp with no other mods - you will get more from head work, cams etc...
Its about the biggest power mod you can do for a fuel injected, N/A engine, the only other thing that will really give you more power is Turbo / super charger which can take the S2ks up to around the 400bhp mark
Wheels - Carbs would kinda be a bit of a step backwards from a fuel injection system, especially with a VTEC system. If you went back to carbs, it'd prolly be a good idea to throw some wild cams in there, and de-VTEC the engine, though I doubt that would make as much power as ITBs, or even the std setup