Greez,
Nice preamp, this is ...
Well, about your questions:
1) you do not need to buy a poweramp at all, the Engl has a "small poweramplike section". but for recording purposes, you do (imo) not even need this ... what is waaay more important than the poweramp (which normally should work quite neutral, concerning MOST modern amp-heads), is a good box&speaker simulation (or something similar) ... the speaker and box have a very noticable influence on your sound, as they work heavily unlinear!
3) pro tools has become my favourite, but only because I had the luck to win a full sized one + library
normally, I would work on Cubase, or ... if money reallyreally matters, audiocity is a nice, and FREE, solution.
Excuse me, if this now sounds a bit foolish ... but have you thought about how to get the signal from the pre into the PC?
(as normal/already included audio devices really suck for recording purposes, imho)?