Don't bother with a MAC - you're overpaying for the same specs as what you would get in a PC, and 99% of your business applications all run Windows.
Quite honesetly, you can get some awesome notebooks nowadays for like $700-800...just check out slickdeals.net and watch the laptop threads come through.
15in is probably the best size for power and mobility and viewing relief...smaller gets annoying on the eyes, larger is a b**** to carry around.
Anything that has a modern processor (like an Intel Core i3/5/7) will suit you well.
I prefer 4GB of RAM just for some headway.
HDD space...well, 500GB would be a good starter point for you, depending on your music storage, etc. The bigger the better.
http://slickdeals.net/forums/showthread ... &t=2268105
http://slickdeals.net/forums/showthread ... &t=2264989
My best advice is buy from a good brand. Lenovo makes excellent laptops, as does ASUS.