The choice of computer should really be decided by which Editor you want to use. Since you seem to be more of a “do everything yourself type shop”, I would stay away from Avid. I say that as someone who makes his living on Avid and Loves Avid. Great editor, but not a great 1 stop package.
Premiere from Adobe (PC) and Final cut (mac) offer much cheaper packaged software that cover all your needs (editing, audio, advanced effects & compositing, DVD authoring) They both pretty much offer the same stuff, Though I think Adobe is a little stronger just because of photoshop. I would go with which ever platform you feel more comfortable because you gonna be spending a lot of time in front of that computer.
good luck
Scott Cumbo
Editor
Broadway Video, NYC