Regarding orks, look at Ramshackle for vehicles.
I'm going to go down one of two roads with vehicles. Roads get it. Vehicles-roads. Well It made me titter.
a) I'll use GW. If I do I'll covert them to a certain degree. I do like the battle wagon but I'll change three things on it. The ram/plough on the front. I do like it and it has a menacing look to it. But everyone more or less has used it. I want a bit of individuality. Change the tracks and wheels. Again going for individuality. Plus the ramshackled tracks are mirrored one each side. And finally doing something about the turret on top. It looks too unbalanced for my liking. Yeah I know it an Ork design but it looks to top heavy
b) kitbash and scratch build.