dont put the PC under the subs, the vibration will kill it in no time unless youve going with a SSD over a conventional hard disc have a look
here 250gb for just £540.
the cases can be small enough to fit under the seats, in a glovebox etc.
Head unit wise im using my old alpine unit. simple got a vauxhall to ISO connector ( this is a 6" extension lead as vauxhall used a ISO cable anyway ) cut it in half and used solder and heatshrink to extend it to just over 6ft. moved the HU to the boot so its beside the PC case ( i used a server case as got it for next to nothing )
Make sure you buy a decent HU, unless your going to run from the on board sound on the PC. to be honest £350 isnt going to get you alot nowhere near the 1500w your after - is that peak or RMS?. decent subs your looking at least £200 each, amps probably the same again, powercap your looking at least 1.5 farad is its 1500w RMS.
to give you an idea heres what i spend on audio in my civic
2 12" sony subs £150 each - 350w RMS 1200W peak
1 alpine class D amp - £200 - this powered both subs.
1 vibe black box amp - £99 - powered the 6x9
2 sets of 6x9s (forgot what they were) - £49 a pair ( were on half price at the time hense 2 sets)
alpine head unit - £250 - was supposed to be £317 but got money off as mate worked there and it was the display unit
kinda scary when you add it up but that was over a good few years
also didnt include the power cap or building materials.
if your going to build the enclosure yourself make the frame out of 1" square and strech fleece over it staple then soak ( really soak it ) in fiberglass resin before strengthening with chop strand mat at least 5 layers.