Read up the sticky thread about buying 2nd hand lenses. The price in ebay can be cheaper, but factor in the costs for shipping and also the fact that you can't directly inspect the item. So shipping back is another potential cost item.
For Canon just use Magic Lantern software, it has focus peaking and focus catch function. Pretty awesome for taking videos.
Contax had their own specific mount and they produced a lot of good glass on license, e.g. Carl Zeiss. As a result one can find plenty of good manual lenses that can be easily used on Canon with the respective adapters. Enjoy, and beware of the GAS