IO540,
If its only for your business email's check out the email software that your using as you can typically set up multiple signatures and you could have one set for each sort of reply you want to send and pick and choose at time of sending.....
Personally I have the Logitech G15 keyboard, although intended as a gaming keyboard it has 6 programmable buttons and 3 memory banks with quick buttons to switch between. Not much use to you I know but rather than pay for shareware etc it may be cheaper to pick one of these up.
Also some of the microsoft cordless keyboards give 5 macro buttons along with the media buttons up the top.
Cant help on the software front tho as dont use any...
Cheers