I would definately recommend replacing the stock springs first. This will help prevent some jarring of the truck from bottoming out when jumping it. (I am going to assume you are going to jump it.) I went with the red and gold spring combination. So far it is pretty good. I still get some MT droop... and it keeps me from bottoming out.
Also, I bought some RPM offset Titan rims and Losi Zombie Maxx tires. This adds a whole lot more than you would think to the truck. You lose some ground clearance and big tire protection (get the springs..) but makes it handle like a dream.
My cousin and I ran our MGTs today, and his kept trac-rolling and flipping over, where I could powerslide and out manuever him. The tires were a big plus. You won't understand until you try it. Definately get the offset rims!!
I already added a Rx for my XS3 pro, so I got the radio upgraded, but I would not worry about doing it first. The only thing I hate about the stock radio is the placement of the 3rd channel switch for reverse. I used to hit it every once and a while by accident.
I have already purchased the forward-only conversion, but I have not installed it yet. i am a little scared of it due to the fact that I read from a guy in another forum who stripped out some of the screws or something like that and ruined the whole tranny. This isn't going to stop me from doing it, but I am going to wait a little while. In fact, I might wait until I get a larger motor. (in the fall probably.)
If you insist on having spare parts in case of breakage... I would get :
a pair of hub carriers. (cousin broke another one today... I have yet to break one)
a pair of turnbuckles....
a set of turnbuckle ends....
check for loose screws after running, and if you notice some that work loose alot, get a spare pack of those screws. ( I ran today with the 2 top screws for the steering cranks missing. My steering sucked today, but it didn't fail me.. can't do this with the ole' Tmaxx. hahaha)
That is all I can think of for now.. Let me know if you want some more info, or if you try something out that I haven't, let me know too...
adrian