I think getting a good mouse and mousepad is one of those things that you can't really appreaciate until you buy them. For example an MX518 is much more comfortable then a regular shaped ambidextrous mouse for me. It allows my ring finger to place in a much more comfortable position which is great for long gaming sessions and even just for the odd 10 minutes here and there. A good mouse surface while not as important as a good mouse in my opinion is still a worthwhile investment. If you have the right combination of mouse and mousepad, the mouse will literally just glide across your desk. So much better like this in my opinion. And as far as my aim in FPS goes, the one thing that has helped me more than anything rather than just lots of practice is to lower my mouse sensitivity so turning requires more movement. It might feel awkward at first moving your arm a lot to move the mouse, but you quickly get used to it. You will want the biggest mousepad you can get for this.
All this considering, the cost of a MX518 and a decent mousepad won't set you back that much considering the cost of other computer hardware. Plus Logitech for example have a fantastic RMA service so if your mouse breaks in the three year warranty period (I think it's three) , they will just send you a new one for free without much hassle. So you can be sure that depending on exactly what you buy, it will most likely last you a good few years.