Orks would be a great army for that sort of thing! Your guys could be a small warband of freebootaz that wander around the galaxy pillaging and plundering, taking equipment from the people that they kill. As they start to get richer and more successful, they start attracting Mekboyz to their ranks who are able to make bigger and better boomstikks, and a few flashy trukks for the army. Then, they run afoul of a Guard army with a Leman Russ...and then loot it. Now with a looted Leman Russ to their credit, the Mekboyz begin to make more and more creative and fiendish designs, culminating in some sort of mega gargant. And then you have a full and proper WAAAAAAAGH!
EDIT: Whoops, yeah, I sorta...went off track there. Sorry. But yeah, do post your basic concept and we can give you some ideas.