OK im just gonna throw my opinion in here
Yes do amp the fronts - very much needed a HU of about 50x4 gives out about 23rms - if that, a good set of comps takes upto 100! youl be underpowering them, and getting little of ther pottential out of them. If you want info on how to do this PM me.
Good set of comps youll need to spend upwards of £150 - lower range of that go for dls a5s - good set about £160, if your looking for sq, i was advised on cystals when i was looking for my most recent pair, ive got some polk momos from last years range, can pick them up for about 200 (rrp 500)
IMO - you dont need to update the rears, rear fill is good, only to a ceratin extent, as it can become more than fill and mess up stageing. IMHO spend more on the fronts
get a 10" sub in the boot in a small enclosure - say a .5cb ft and a 4channel for everything say the mac attack 4075 (v good power rating are underated), 2 channels for the sub a channel each for the fronts, standard rears off the HU
As HU wise i would go for the 7894rb - last years top of the range alpine - awsome unit rrp was 500 - mcs do it for 275
Ive based this on a sq setup, on a medium budjet of about £8-900. I coved it a bit in the thread - proposed ice - for any more info PM me.