WOW. Just wow. The only way you should even THINK of a mini-t is the mini-t pro. Go the 18t route. With a few select parts, it's a GRRRRRRRRRRREAT! truck.
I have both. The RC18T is better than the Mini T hands down. It's better balanced and it's 4WD. Whatever your driving habbits are; down the street, accross the lawn or into the dirt you'll have more fun with it than you will wit a Mini T.
Shim the diffs are optional but highly recommended. A better battery pack will give you higher performance.
Yes. Driven pro drag-link, spring retainer mod, shimmed diffs, and I reccomend a hex-heads conversion. I would also advise you to buy an rpm bumper and get the factory-team kit.