Front speaker door covers just pop out (assuming there the same on all 350Z's).
Rear grills dont come off and involve removing half the interiors trim to get at the speakers.
If your after a cheap and cheerfull setup then i reakon £200 for a headunit, £100 for some front speakers, i wouldnt spend more then £20-30 on the rears.
For under £200 this looks ok...
http://www.caraudiodirect.co.uk/alpine-cde-w203ri.html
Cant go wrong with alpine in my opinion.
As you already have an aftermarket headunit you shouldnt need any extras, if you did the fascia, steering controls adaptor and harness come to around £60.