<sigh>
It is MUCH easier and it would sound INFINATELY better if you used real home stereo gear.
For Example:
Source:
Cheaper 'Quality' Car Head Unit - Such as Pioneer $350 vs Denon Low Range CD Player ~$350
Low range Denon etc would still sound about one trillion times better than even most uber Car Headunit's. It has the advantage of pure audio orientated DACs and circuitry (ie not also a receiver) as well as much larger and constant power supply. Also, the actual chips themselves would be of higher quality and the player would have far better EM sheilding.
Amplifier
Decent Car Amp ~$500 vs Decent Home Power Amp ie Behringher EP1500 ~$650
You said you were looking at running a couple of subs AND a couple of full range speakers, so you'll need a decent amp of around 200WRMS a channel (or a 4 channel@~100). The Behringher EP1500 (what I use) is 450WRMS a channel@4ohm and can go down to 2 ohm. That is enough to run 4 x 12"s and dual Piezo Horns at around 120Db free air. As a side note, the 'Old Days' when you couldnt run car speakers off home amps, but those days are gone. Once again, the home amp will sound better as it will have far less distortion (at even higher levels) and also they have a very beefy power supply (which again is inportant for sound quality)
Speakers
Quality Car Speakers (2xFull range and 2xSub) ~$800 vs Good Quality Home Speakers ~?
Now this is a hard one. Decent quality speaker's for either type of system can cost ALOT of money. For example a pair of good Pioneer's retail at around $500 and as for subs, I have no real idea on something like Alpine Type R's (most people REALLY like them) but $200 each wouldn't be out of the question for decent ones! So that's around $900 or so. On the other hand, assembled speakers for home start at around $500 as well with subs costing lots...and lots... starting around $500 again (powered however)
The other option is to build your own, I built mine for around $650 but I was lucky with the drivers. You could go with something using good 12"s and a good piezo and you probably would NEED a subwoofer if you designed the boxes properly, it depends to what you listen too, I'd like a sub for when I get the feeling to listen to some Goa but generally, mine are more than enough. There is always the option of using car drivers, like 6x9's in a proper box and I'd say they'd sound pretty good, at least better than unboxed in a car...
They are just my thoughts on the matter. It depends on how much you want to spend, how often you'll listen to it and what kind of music you like as to how much you want to spend.
However I can catergorically state, IF YOU WANT TO LISTEN TO MP3'S, GO TO CRAZY PRICES AND BUY THEIR SHITTIEST ALL-IN-ONE DVD PLAYER YOU CAN FIND save your money and spend it on something useful...like Red (or Green) frogs.... <shudders at thought of MP3's....>