I don't do much scanning myself (yet--I do have a pile of books that's starting to bug me), but two things come to mind, especially at your price point. The first possibility is to go with anything that has a large scanning bed. The bigger the space, the less you have to shift the pages around.
A similar alternative would be a sheet-feeding scanner, which has the same benefits and might run faster, but sounds like a potential danger as the grippy rubber wheels push your fragile pages passed the camera.
Otherwise, I suspect the machines are all interchangeable. The motors and gears are probably more fragile in lightweight or slimline models (mine sometimes slips when resetting for the next scan until it warms up, which is annoying), and anything not dirt-cheap presumably doesn't suffer from the "6MP Limit" that small digital cameras hit (
http://6mpixel.org), so I would think you just want the biggest, heaviest model you can find.
But that's a VERY theoretical suggestion, and you probably want empirical data to back that up before pulling out the plastic.