If you will not be using the car primarily for track driving, don't bother with coil overs. Adjustable height may sound cool, until you realize that you have to pay for an alignment every time you mess with it. A conventional setup will give you everything you need for a street car, for much less cost and complication than coil overs.
The 318ti shares the same front end suspension with all other E36s. The rear suspension came out of the earlier E30- so E36 rear suspension components will not work.
Bilstein Sports are a great option for lowering a ti, but you of course need to make sure you're ordering ti-specific rear shocks. They can be had at
Bavauto, among other places. Bav also sells a full set of shocks with their own lowering springs for the ti for less than $700- a pretty good deal. You'll love it, twisty roads and on/off ramps are a real blast.
When you replace the rear shocks, make sure you add reinforcement plates for the top shock mounts. BMW makes OEM re-enforcers for the Z3 that work perfectly in the ti for about $20.