ah the good ol 'scanger my banger' disease, everyone catches it at some stage!!
i just bought a pair of 70w Sony 6x9's for about 50 quid and hooked them up to a fairly standard Sony CD head unit. it's not that hard to do but will depend on the connections in the car(connected to the existing radio). you will use connectors called ISO connecters, they may be they already but may not(depending on how old/new the car is). if you do not have these, prob best to let the experts at it, messing with car's electrics can cause fire's etc.
edit: 50-70watt speakers for the rear (6x9's) are definately sufficient. don't bother with amps and all that shite. maybe 40-50watt tweeters for front would finih it off. general opinion is that Pioneer's head units are the best. don't know who best with speakers but from my very limited experience, sony's are good