...you're not related to moahdib, are you?
Anyway, I don't see anything wrong with not having a head unit in the car, I still have mine and do not use it with any frequency...and I'm very new to the hobby. I'd recommend an outboard audio processor/crossover in place of the HU, and take sound to it digitally from the PC (so as to eliminate possible noise from the connections to the computer and all the peripherals)...a processor will offer you a flexible means of providing crossover, time alignment, EQ, and a host of other goodies to whatever amp/speakers you choose, and if you look around, you should be able to find them priced in accordance with what you intended to spend on the head unit. Store your songs in lossless flac or ape, and enjoy access to ALL OF THEM AT ONCE, rather than fumbling around with CD's. Choose a bit-perfect capable soundcard like the M-Audio transit, or a similar DAC, connect to processor via optical/coaxial digital, and you're golden wrt to everything prior to your crossover.
As far as amp/speakers, I'd flip over to elitecaraudio or DIY mobile audio for research. DIY has a great install section, perfect for ideas...between here and there, you should be able to get all your questions answered.
As far as using the head unit in place of an amp, this is almost never a good idea wrt to quality sound or power handling...I'd almost recommed sticking with the stock headunit/speakers rather than spending the money on a potentially regretable "upgrade". Only do so if you plan on getting high efficiency speakers, or keeping (after researching the feasibility) your stock speakers in place. As a good rule of thumb, assume output power quoted on a head unit to be 2-4 times actual RMS power (they typically use a different class of amplifier than you normally find in outboard current amplifiers used for car audio aps).
I'd wholeheartedly recommend a simple soundcard - amp hookup as more robust and flexible than a headunit powering speakers (but then, take my opinion with a grain of salt, I'm very picky and there may be good ways of powering a speaker set from a head unit nowadays...I just have yet to hear of anything in this regard that I would do with my system).