First off, talk to your neighbor about it, because if they know this, they may put off a potential buyer for a bit of time to allow you to see if the bank thinks you can swing it, otherwise they may get an offer and take it, never knowing your intentions, and you both may end up feeling bad about it - not with each other, but that "man, that would have been nice" - regretful, not angry, I mean.
Second, get a move on man, shake a leg, GIT 'ER DONE! If it's something you want to do, you will move on this at a pace that will show your interest, and it sounds like you're interested, so I believe stepping up the pace in seeing the bank is your best move.
Third, find out what he wants to sell it for, and if it's agreeable to you, talk to him about the rent situation so he won't have to make the double move.
There IS no good way to bring this up, other than to be straightforward. My wife and I have to do this EXACT thing later on today with a family member, and while neither of us is looking forward to it, if we never talk it over with them, we'll never know if it's possible to do, or whether they were even aware of the situation.
There doesn't exist a way to talk about a tens-of-thousands-of-millions-of-dollars transaction easily

Brother, you and me BOTH wouldn't have that little pitter-patter in your heart just THINKING about it if there were!