Wow, it's going to be hard to answer this sufficiently but I'll give you my quick .02:
1. Get more than a 4-channel mixer if you are playing live (4-bus is greart, 4-channel not nearly enough). I just bought a Yamaha MG12/4 for $219 new from Zzounds.com. You'll be able to plug in turntables, drum machine, mic, synths, whatever with this one. Bang for your buck its a great mixer.
2. The Virus Ti is a monster synth and can do whatever sounds you need it to do. The keyboard is second to none if you get the full 61 key version, but it is not cheap.
3. Drum machines have been a real pain in the arse for me. I haven't had the cash to spring for an MPC or a machine drum for the last 6 years, but I have been able to buy a M-Audio Trigger Finger and FXpansion's GURU (I don't know if they make a Mac version, if not just get Stylus RMX, it's just as good). You can make some bad ass drum tracks with this combo, and get it for less than $400 for both if you shop around. If you want a drum machine and can't spring for a top of the line new one look at a used Roland SP series or maybe even a Korg Electribe of some kind...
4. Don't buy an M-Audio Oxygen. Don't even piss on one. Spring for a nice Novation Remote 25SL or even better a Remote 37 SL. It will auto map to your sequencer and to your soft synths so all your knobs and faders are linked to soft synth/drum machine/sequencer parameters as you click from VST to VST. Pretty fucking bad ass if you ask me. It also has drum pads on it so you could not even buy the Trigger Finger if you get either of the Remote SL controllers.
5. Don't forget a nicew audio interface. You have a Mac, get a MOTU if you don't already have one, they kick ass!!
Good luck, I hope some of this rant was helpful...
-Alex