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I'd look at one of the one of the powermaster primes coming out. You'll get a little dude that connects to a truck, a truck that turns into a robit, the robit that connects to his trailer for a super mode, and then another trailer armor that connects to it on top of that. Perfect Effect's got addons to the recent official PM Prime, THICC Hobby and Maketoys also have one in the oven. I guess Iron Factory has a legends one out already. Avoid XOVERGEN like the plague.
I would also look at grabbing Fansproject's Roadbuster/Roadbuster twins. They came with armor bits that could completely change the silhouette of the vehicle modes and bulk up the robot mode. You could probably find one of the twins for cheap without their armor, then buy Shadow Fisher's upgrade set which is just an imitation of the armor they come with but with a few upgrades.
