I suggest you spend some time at the
www.mynewavalon web site. It will answer many of your questions, including some you haven't asked yet.
(1) The NAV is a subjective question. Some will answer yes, others no. To me, the positives include a nice large display in a convenient location for the driver, and reduced likelihood of theft. The negatives include several annoying bugs that Toyota will neither acknowledge or correct, that you sacrifice continuous display of audio and temperature information when the nav is in use, and that many nav features are locked out when the vehicle is in motion.
(2) Thw bluetooth is only available in certain regions of the country, added by a distributor. I don't know if nav is required for bluetooth. I suggest you do a search for bluetooth posts on this forum, toyota nation, and
edmunds carspace forums.
(3) Yes. The front seats in the Limited are both heated and cooled/ventilated.
(4) I think the answer is yes. I don't use the MPG computer. It is just as easy and more accurate to keep a running record of the trip meter reading and gallons purchased each time you fill the tank.