for ICE, probably the best value system-design include:
*2-way 6.5" components for front
*single 10~12" sub
*4-channel amp: channels 1-2 for fronts, ch.3-4 'bridged' for sub
(*retain stock rear speakers)
hard for me to quote prices cos i'm in australia, but, presuming a rough 1:2 ration of UK:AUS, i'd estimate:
*CD HU UK$100~200
*component speakers UK$200: focal 165V1, boston acoustics SX60, hertz HSK165, mb quart DSG-216, morel pulse 6, digital designs DDC6.5
*sub UK$100~200: alpine type-S/R, boston acoustics G1/2, JL audio 12W1v2, image dynamics ID12v3, digital designs DD512
*amp UK$200~300: alpine MRP-F600, audison SRx4, digital designs C4a
note, i cannot say for sure that those speakers will drop into the front doors without any mods. could require minor or major mods, from trimming the plastic, to cutting that section out and replacing with an mdf wood spacer ring. though if you're paying a pro to do the work, this would not be a difficult job.
for a quality entry level sub as suggested, you can make do with a prefab 'sealed' enclosure. though you're going to have more optimal performance with a custom designed and built version, especially if you go with a 'ported' enclosure (those subs above suiting ported are the alpine type-R, ID ID12v3, and DD 512; though each work fine in sealed boxes).
also factor in all wiring/cabling, which can be another UK$100.
for a HU:
*alpine: great for iPod, features, value, but BT adaptor (ie: KCE-300BT) is buggy and far from best on market; probably better to buy a Parrot CK3100 kit instead
*pioneer: those with BT inbuilt are very good from all reports; new '08 models look nice and sleek; 'blue' display doesn't suit alfa's though
*blaupunkt: some have inbuilt BT
other HU issues you may priorities include:
*steering wheel control compatibility: if applicable
best to find a specialist car audio store and discuss your budget and options. you can always do it in stages, starting with a quality HU + component speakers; stage II can be amp + sub. if you buy good quality gear, it can last years and years and go into your next car. some of my audio is years old, still going strong, and would cost double to have worthwhile upgrade/replacement. so, buying wisely - rather than cheaply - can be a reasonable investment.
for a bodykit, i had a full Zender version on my 156 and i loved it. i didn't go with the high wing or deck rear spoiler, but chose the rare lip spoiler that is flush and simply sticks back. high quality ABS resin material, not fibreglass (not sure what the Cad one is; but FG will crack and split if you nudge a curb/driveway hard enough).