Awesome idea on upgrading with a starter box from the get go... After you get away from pull starters, it's near impossible to want to go back. Gerwen had the right answers, so no reason to reiterate there.

.28 will be good if you are bashing, if you want to hit the track, I would stick with a race legal .21. One thing you may or may not know is these buggies are much lighter and much faster then an MT. They don't need the monster displacement engines that the trucks do. They are rockets.

A 21 will give you better mileage to boot, and the difference between a good 21 and 28 isn't as big as you would think. Just food for thought.