First of do you like low heavy bass or are you more into high sound quality?
£600 is a healthy budget if you ask me if you do shopping around you will be surprised what you find.
If it was me the first thing i would buy is sound deadening for your doors and if you want to do more of the car feel free but the price will rise as you need quite alot.
Alot of people may disagree with me but i wouldn't touch alpine i find them pretty good if you dont know how to set up a stereo to how you like it but my personal choice in headunits is pioneer.
oh yea avoid halfords unless you want to waste money
I am fully with you on this one, I do like alpine head units but if you really want a good head unit a pioneer blows anything clean out of the water!
Sound deadening is a brilliant thing to fit into your car, less road noise and boosts your speaker sound tremendously, I run a very good a clear system atm but I spent a lot more that 600 quid but I'm very big in top of the range audio, if you want a nice simple and fairly cheap system I would do this:
Front and rear speakers: Vibe Slicks - Very clear in the highs and mid range and very good bass to the speaker if it is needed
Sub: Personally I would spend a third of my budget on a sub, I have a kicker L7 in one of my cars, square sub, more surface area, extremely well mad, fair cheap if you jump on eBay
Sub box: Easy, go on eBay again, custom built, 12"-15" box, PORTED!! I can't stress this enough about how important it is to get a sub ported, a sealed box is pointless, lose around 70-75% of the bass, a slot port is ideal for something like a kicker, gets the air flowing and the bass clear.
Sub Amp: Tricky one really, dependant on the sub you buy, I use loudest.com amps, specialised company, amps are made extremely well, couldn't ask for anything better really...
Amp for door speakers: If you really wanted a loud car, the vibe slick amp wired to the 4 door speakers, have it set to just handle mids and highs, the sub can run the lows
Tweeters: Every car I have ever had which has had a full setup in I have cut the tweeters out, removed them and never replaced, I find the tweeters on the speakers handle the highs just as well and therefore require no more assistance
Headunit: Depends whether its a single din or double din, I also got for something easy to navigate around but something you can control all your sound with, something with an equaliser (that actually works) is perfection, you can handle everything on the stereo, nothing easier
Thats my two cents anyways, hope you find this post some use, if you want any further help message me as I'm more than happy to help and see whats going on within ICE
P.S Do
NOT go near halfords, they no absolutely f**kall about a good car setup, you go there if your a chavy d*ck who wants there car to sound like there is a constant thud coming from inside of it, avoid at all costs!!
Enjoy