McCain has already sworn (repeatedly) that he would hold only one term (same promise he made back in 2000).
How much credibility, and trust you have in him is up to you, but legacy is not part of the picture if he is telling the truth about his plans, if he really wants to follow through on Bob Doles Promise (in 1996, he also promised only one term if elected), then the VP candidate becomes the major consideration, and I don't trust McCain nor know who he would choose as his VP.
The talk is Huck as VP which would make sense since he’s a fellow fiscal liberal. One line of thinking is to take McCain and wait for 2012 where the census has people moving from NY, MA, PA, MI, IL and other blue states to FL,AZ, TX, UT, NV and other red states.
The R party seems to elect the next in line whether it’s Dole or McCain. Next in line in 2012 would be Mitt.