Well, for cost effectiveness:
1) Beg all your buddies for leftover tank bits they don't need.
2) Buy crappy looking old tanks off ebay or at store auctions. The worse the better as they are cheaper. Half primered, too much glue, put together by a dislexic 7 year old on speed, missing treads? It's just raw materials.
3) Big bag of balsa wood scraps, and bits order the little sprues with orky wheels, and the iron looking wheels on the big guns.
4) Break all the bitz up. Make a rough frame from Balsa. I start with a piece for the truck bet, two 1/4" squar pieces running across like axes to glue the wheels onto. Then I make an armored cab out of bits, more bitz and balsa for the sides of the truck/buggy, and add gunz to taste.
5) Primer it black, drybrush with tinbitz and then boltgun, next add lots of red and yellow racing stripes and orky glyphs. Have enough red to count as "red paint job".
Plasic ork boxes are your friends, and you can mix in a box of plastic fantasy orks. Leftover bitz from a fantasy ogre box are great as well to mix in.