Ok, first of all, you will need to check the engine to see which one your car came with.
The 97 and early 98 Z3 came with the single-vanos M52 B28 where later 98 Z3 came with the B28TU (technical update) that features the dual-vanos and different intake manifold.
The TU engines are more difficult to tune and thus there are fewer kits/upgrades available.
If you have a non-TU engine then the 3.2 (m3) and 2.8 (328) kits will work on your car (with the exception of some piping that may need to be customized)
There are numerous tuners of FI kits:
- Active autowerke
- ESS tunning
- VF engineering
First thing is first, like I said find out which engine you have on your car.