Hmmm, those questions seem pretty amateur..sorry dont mean to offend, just couldnt think of another way to put it. Since you are asking these types of questions, it basically means you have no idea what you would be getting into. You will have major issues with all the things you mentioned.
Can you imagine an S52 BMW engine mated to a 30+ year old datsun 4-speed transmission? Makes me laugh. You have A LOT to learn.
(by the way, you cannot give away a 4-spd datsun trans, thats how crappy they are...) If you do get a 510, just do some simple bolt-on mods, maybe swap in a common motor like a KA24 and just take it step by step and you will learn.
Or, instead of blowing all your money on a swap that would never see the light of day and end up getting sold off half way through, just buy a 510 with a motor swap already, like an SR20DET.
Nissan by far has the best selection of hot 4-cylinder RWD engines of any manufacturer. And they all can make plenty of power to make a 510 really fun.
Forget the inline 6's, the 510 has a super short engine bay, any six cylinder is going to go way back into the firewall and your shifter will be in the back seat and you wont have any room for your pedals because the bell housing will be right there. It would require major metal working, custom parts fabrication, custom wiring, custom engine management, etc. ... And since no one has ever done it, you would be completely on your own. Expect to be into it at least $15k and 5+ years, I guarantee you would lose interest in it because the problems would be so huge and expensive.
If you really want a car with a BMW straight 6, then swap one into a E30 M3, heck it is just like a 510 (kind of....). And I have seen pictures of it done.
