If you haven't tried it, I strongly suggest borrowing another monitor just to see the options. How often do we follow a guide on a website while attempting to do something? Browser one screen, actual work on the other. I find the desktop real estate indispensable, but I guess to what degree depends on the user.
My BestBuy suggestion was so that you can actually hold the mice you're considering before buying. Brick-and-mortar stores will often have mice and keyboards on display. You will still probably find better prices online though.