I've been dusting off my orks lately and I have an old Gobsmasha I've been using as a Gunwagon with DaBooma and had some fun with it. It's a bit of a conflicted unit as it has that transport capacity of 12 so it kinda wants to be used to haul something but with the double shots for moving slow it kinda wants to NOT charge across the board like a Trukk, plus it's not open topped so you can't use it as a bunker for Lootas or Flash Gitz. It's tricky.
I'm using it in a crusade campaign so I took it with the big gun and two big shootas and then put my warboss, painboy, and 5 mega nobz in it to let it move slowly up the board behind a wall of boyz and under the protection of a KFF. It lived until Turn 4 - most of the army did not - so there is that, and it dished out some hurt.
I think the weapon choices are fairly straightforward, the tough question is what else do you want it to do?