The z31 aftermarket is pretty darn limited. The main place you're going to find parts is
Welcome to the Acadiana Sports Car Orphanage! | Acadiana Sports Car Orphanage - Official Store
And for suspension, you're also pretty limited for choices. There's your standard strut/spring combo's, and for coilovers you're prettymuch limited to Stance, and now Powertrix.
Steering, not a lot depending on what you're looking for. Gary Molitor does limited runs of steering rack poly bushings, and there's a group buy for higher angle steering spacers right now on z31performance.com. In terms of steering wheels, the s13 hub's fit for quick release or aftermarket wheels.
For lips there's the SS or TBO replica lips, or if you're balling out of control, which I assume you're not being that you bought a z31 (
), there's Proudear out of japan.
MSA has body kits as well, most of them are ugly.
As for swaps, depending on what you're doing with the car, if the car is turbocharged now you probably don't have to swap engines. If it's non turbo, the easiest thing to do is swap the turbo bits onto the n/a engine. If you're set on a swap, the sky is the limit, but the car came with an rb20 in japan (200zr), so that's probably the easiest swap. Aside from that, vg30dett is ok, vh/vk is possible, or whatever else depending on level of expertise and money.
brakes, you can bolt z32 brakes onto the front if you have 87-89 turbo hubs, there's a member who figured out how to do it with the older hubs, but you'll probably want to swap to 5 lug anyway. There's lots of options for BBK's, they just take some research and doing to get em to work.
This is pretty good basic reading
TURBO FAQ: MODIFYING A SERIES I 300ZX TURBO FOR SPEED
also check out z31performance.com for a decent group of z31 owners. just avoid that DeleriousZ guy, he's a dick.