I must say the MX518 is among the best mice in the world, and until the new version of the G5 came out, I reckon it was.
But I also believe that the best mouse in the world is the one you're most comfortable with, so it is very important that you actually physically try it out. At least have a feel of it. The best thing you can do in this regard is actually get hold of one, and use it for as long as your longest gaming sessions. Which means it can be upwards of 36 hours straight for some people
If you're perfectly happy with it, with the number of buttons, feel, etc, and have no grieviances (other than price), then its the perfect mouse for you.
As far as mousepads go, again, its the feel of it. Some people might not be very interested in a super-slick mousepad, and others cant have anything other than totally unrestricted movement. Size is another important factor (I've seen people playing CS requiring 1m of mouse space

. Can't say they were good at the game though...). I was used to having a hardcover textbook back in highschool, and thought there was no use spending money on a mousepad, but my latest acquisition (eXactmat) proved that spending money could actually enhance your gaming experience. Or at least reduce the number of times you missed that headshot due to the mouse not being slick enough.....
All boils down to personal preference.