Need some motor advice!!!!

[SWATsn1per]

I bought my fiance a 3905 MAXX for Christmas and she loves it. We went bashing today and she loves the fact that it is a do-all go over anything truck. I was thinking about getting her an MMM like I have in my E Revo. The only complaint she does have is she goes through NiMH packs like crazy. As far as runtimes go she gets about 15-20 minutes on a pair and she sees me going for 45 minutes on my lipos. So, that is when
I though about upgrading her truck to brushless too. What is a good motor to add to it?
-First things first is she does not care about speed. She can go 20mph and be perfectly happy. So a fast motor is not needed.
-She just wants longer runtime. I know longer runtime will come naturally when the upgrade to lipos is made but I want some suggestions from some of the experts.
-We will be sharing lipos so I need a motor that will run 4S and 6S.

Thank you for your help.