I have a Vehicle heavy Imperial army (filled with Necros and bikes) so I speak from experience.
Recommend looking into proxies.....(necros are hard to find and not the greatest looking model)
any small off road car/buggy with a machinegun will do.
check these out
http://theminiaturespage.com/news/450615/not 100% sure of the size but they look like it might work.
Gw Ork buggies could also fit really well
Void has/Had Buggies that work perfectly (I use these myself)
also very hard to find but a line of minis called Future War had buggies/ cars that work nicely too (I too use these)
sometimes you'll find them in the discount bin.
Check plastic historic model kits theres few in the WW1 and WW2 that can fit very well with little coversion work (yet someting else I use)