As far as I know, there are 3 different racks; manual, power and a quicker ratio one on the Euro M3 (maybe the US M3 too, not sure). So you can take one of any normal E30, the 318is mostly have power racks I think.
I have a power rack for sale, but I'm in the UK

If you can, you should think about fitting an E36 or Z3 rack though. They are a very good upgrade.