I used a starter from an E28 535i. I got lucky and found one lying on the ground at a junkyard underneath the car. The bolts are difficult to get on, particularly the obvious one. I don’t remember if its 17mm or 19mm, but it is one of those. I highly, highly recommend installing the starter prior to dropping the engine/tranny in the car.
Don’t forget about starter wiring. You can do this after the engine/tranny has been dropped in the car. You will need the obvious power wire. You can use the one from your E30 if you like, or get a 2 gauge wire from homedepot or something. Make sure that the loop on the end is clamped on bigtime if you do the homedepot route. Now on to the other wire for routing. You will take the black wire with the yellow stripe and wire it to the starter. Remember that the starter is already grounded. You already hooked up the power. Now you need that 12V “switch”. That ignition switch wire is the black with yellow stripe and you will see on the starter, there are two tabs to attach that wire on the solenoid. Read carefully as I show you which one to mount it on. If you properly install the starter, and you look at the driver side of the engine, you will mount the wire on the spade terminal that’s on the top of the solenoid. It’s the green arrow pictured below.
The E30 ignition coil is fine for the swap. No need to use the E28 coil. You can change out the spark plugs and wires if you wish to as well.