Question 1: No. If you pick options by their preset (low, medium, very high), the bump-mapping will not be enabled for the lowest or medium. You can, however, select the features you want enabled via advanced in options.
Question 2: Surprisingly, bumpmaps don't sell for as much as people think they do. Full bumpmaps are worth around 41k at the moment, but if you're buying bumpmaps individually, it would be best if you bought them at 1k or so (but you'd barely make a 200 TC profit. You might be able to get more from those who think they're worth the retail of 5k, but that doesn't work in most cases. Individually, bumpmaps are sold for less than 128x128 textures. As a whole, bumpmaps are sold for more than full 128x128.
Question 3: Marcus pretty much got this one. Basically, it adds depth to the applied body part.