How long the motor lasts is completely up to you. If you do a PROPER break in procedure, and you perform PROPER maintenance on a daily basis such as changing the air filter a lot while out bashing, running a high quality air and fuel filter, and unsing the proer after run procedures, your motor will last a REALLY long time if it's tuned correctly. However, not doing some of what I mentioned above could cause your engine to last only a few bash session.
The rustler and the T-Maxx are two completely different animals. The rustler is more of a 2WD stadium truck style race truck. The T-Maxx is a 4WD monster truck that's more oriented for bashing and handling rough terrain. The brand new 3.3 T-Maxx has a lot of improvements over the regular 3.3 T-Maxx, so keep that in mind when you make the purchase.
What are you going to be using this truck for? That would help a lot. If you like the idea of a rustler but you're going to be bashing it, you might consider a stampede. Very similar, but it's a monster truck style truck so it sits up higher than the Rustler and has some different characteristics like that.