4.10s are not just going to give you acceleration. They will make the gear changes shorter, so you will be shifting more and the car will feel tighter (more like a motorcycle if you have experience with those). Just want to make sure you understand the change, as it does change the way it feels and how you drive it.
As for the diff. Find a good diff builder. Doesn't have to be bmw or any brand. Just look around until you find a diff builder with stellar ratings, even if you have to travel. Best most cost effective way of doing this mod is to buy the gear and pinion and have it installed in your own diff, but you need an excellent diff builder.
Look around for offroading / hot rodding / etc. as they tend to do this kind of work. Look at
this thread for more details.