Don't do it. Cameras and lenses are a low margin, high labor industry. Professionals are exceptionally picky about their gear, so you need skilled technicians working hard to keep your gear in top shape, and the prices are very low.

The bigger rental houses make all their money on other things -- the ROI on sandbags, sound blankets, even consumables is ridiculous compared to cameras. (Granted I'm blending grip and camera, but some houses do both.)

You're also competing with owner/operators, and even in-house studio operations. It doesn't scale -- relationships matter and it's highly regional. Compared to other ways to put your capital to work, it's not a great investment.