1) You will almost certainly find it impossibile to find an "off the shelf" exhaust for an S12 so custome all the way ie; Powerflow, Longlife etc.
2) My S12 wasn't especially "turboy" it just kinda has a second wind around 1500-2000 revs but it's a tiny turbo and not running all that much boost. Not to mention 130bhp from new (so probably about 100 now)..
3) You're only real option is either some custom affair or BC Coilovers. BC are actually pretty good but yeah you need S13 hubs. The rear is a seperate adjustable dampner and a regular spring with a machined adjustable seat, similar to the adjustable sections of a coilover.
People arwe always like "OMGBBGCOPTER teh shit etc" at the S12 IRS but I don't see why they moan so much...