You are playing rock music. Nothing you get in the way of sound modules is going to sound like a real band.
You can use cubase, logic, cakewalk or any sequencer for playing midi files with total control over individual tracks and route the midi to any external sound module.
All the above programs have audio capabilities as well. Try doing a search for midi sequencer and you may find something cheaper.
Forget soundfonts, they are for soundblaster cards which are cheap and amateur.
There are many software samplers which you can buy high quality samples for.
I would be very wary of running your whole live rig through a computer. If it crashes it won?t look good.
The stage is not a nice place for a laptop. Heat from lights, danger of spilt liquids, knocked about by drummers...
If you have a rack of external sound modules as opposed to soft synths you can actually buy a midi file player, stick that in the rack and leave the pc at home.
Could be a bit more reliable whilst on the road.
You may get some better advice from a live sound website rather than ones devoted to recording.
Hope that helps.
