Something like the G600, or a similar mouse with many buttons (search Amazon for 'MMO Mouse' to see others with similar layouts) might be a good option so long as you could handle movement controls with your left hand. I'm not sure that array of mouse buttons would be suitable as a replacement for standard WASD movement controls, seeing as they all need to all be controlled by the thumb, but you could assign them to most other keys that a game uses though. One nice thing about PC gaming is that games usually let you reassign keys to whatever works best for you in the options, and mice like that are also likely to come with software that allows you to set their buttons to any keyboard keys. If you have trouble using keyboard keys for movement, since you may need to hold down a couple at a time, using a joystick for movement in place of the keyboard might also be worth trying. Even if a game doesn't natively support using a joystick and mouse together at the same time, there's software (like JoyToKey or XPadder) that can make a joystick emulate the WASD keys, among other things.
If you found that you preferred to use your left hand with a mouse, and your right with the keyboard, then a many-buttoned mouse might not be the best option. But if you normally use your right hand for things like writing, then it would probably be best to use that hand for the mouse as well, and assign as many keyboard inputs as is feasible to it.