When I first got my car it has some major problems going into 2nd gear at high engine RPM's. I would either have to skip 2nd all together for the first little while. For me I thought maybe the synchro ring was gone but funny thing is the transmission works just fine now. It probably adapted to the previous owners driving habits so to speak. These synchro rings are off of an E30 (can't find any pictures of the ones in the E36's ZF transmission at the moment):
Above images all from Google (cartechbooks.com)
Fluid can also be a huge part of this too, of course. I have actually heard more instances of people flushing the fluid and not putting the right viscosity in and that can cause the vehicle's transmission to not go into gear etc. Pelican parts suggests this stuff here:
"Swepco 201 is the gold standard for transmission oil and is used by racers and BMW owners worldwide, many of whom swear it gives new life to old worn-out transmissions. Customers’ stories from the parts supply world seem to indicate that using Swepco on a transmission with worn synchro rings can prolong its life and fend off that costly transmission rebuild."
BMW claims the fluid is a "lifetime" thing and is best left alone but so many apparently tried that stuff above and had fantastic results.
The people at the dealer were shocked to hear how my car sort of repaired itself and works perfectly fine considering the issue it had right after buying it in 2010. So a completely replaced unit is not always the remedy. Plus if you went that route you could sell yours to be rebuilt and purchase a properly rebuilt one and have it installed for around $500 or so at a shop if you don't have the space to do it yourself. Hope this helps a bit.