First, the avistar is a great kit, you'll be happy with it for a while. To answer your questions....

Yes, the 3151's will work, but honestly they are overkill.... The 3151's are BB and digital.. neither of which will do you any good whatsoever on a trainer like the avistar... If they are what you have, then use them now, and when you "upgrade" to something more precise like a pattern plane or something like that, move the 3151's to it and replace the trainer servos with some standard $10 servos...

The avistar will work great as a trainer. It's a really great flying plane... Tower says it's "advanced" because it has a semi-semmetrical airfoil... It still has plenty of lift and dihedral... It'll train just fine, but you won't get bored with it NEARLY as fast as you would a standard flat bottom.

I've never really looked at a trainer as being "worth" modifying, unless it's already disassembled and needs to go back together anyway.. Seriously though, follow the instructions (they are pretty good) and you'll be fine... The only thing you might consider is replacing the stock push-rods with some nice sullivans or something.... I like "gold-n-rods" myself... The wire ones are not only a pain to work with, but they'll also never be as smooth as plastic.... The throttle servo is fine though.. you want to keep the wire pushrod for it.