I know 17cm component speakers will fit in the doors in a mk4 golf but I read another post mentioning an adaptor being needed for them.
Amp wise, if you get a 4 channel amp, bridge the channels to create 2 channels, bridging is just a term used for wiring the channels in the correct way for the amp you purchase to create more power

I've advised a 4 channel amp just in case you want a sub, these then leaves a nice easy way to add it, as all you will need to do is get a sub, a box for the sub and connect it the amp and it's done.
If I was in your shoes I would set myself a budget of around £100 for a 4 channel amp and
£100 ish for the front components
A wiring kit will cost you about £20 and you may need additional speaker cable at about 50p to a quid a metre depending on where you can source it from.
Here are a few items I would look at:
Amp - JBL GTO 75.4
http://www.caraudiodirect.co.uk/product_info.php?products_id=4140They are the kings of budget car audio and you get a lot of product for the money!!
They also do very high end gear which I haven't seen in this country and it's meant to be the dog's watsits.
Speakers
You will need these adaptors to fit aftermarket 17cm speakers
http://www.caraudiodirect.co.uk/product_info.php?products_id=517You will ideally need to have a listen to a few sets and make a decision.
Amp wiring kit
You could use a 4g kit which gives you a bit of room to upgrade without needing to upgrade the wiring
http://www.caraudiodirect.co.uk/product_info.php?products_id=204or an 8g kit
http://www.caraudiodirect.co.uk/product_info.php?products_id=206for the sake of a fiver go with the 4g
And finally you might these headunit adaptor but not sure, I haven't worked on a mk4 before.
http://www.caraudiodirect.co.uk/product_info.php?products_id=654I have a friend who is a distributor of European amps, speakers, subs and wiring.
I'd like to say we could help you install it and also provide you with everything you need but I know he is a busy guy and has a baby on the way + it would be an install as though a mate helped you with it, not done like a shop would where they would be insured for the work they do, guarantees etc.
If you go to
http://www.innovationaudio.co.uk and have a look through the products, email my friend (email address on the site) he can quote you for what you need. My whole install is based around the higher end of the products he supplies and it is nice kit. I'm not just saying that because he is my friend

Tell him James sent you
