Answers to the questions the guides can't answer

Hey all, i have been lurking around this forum and the internet reading all the guides i can and planning a new ICE for my 1.2 fiat punto mk 2
SO, i was wanting to replace the front speaker and add in a sub, replacing the head unit will be necessary too, because it's a tape player. I have never really fiddled around with cars before, as this is my first one and i could really do with some advice!
Looking around websites i found these speakers, which fit the front door pannels of the punto
Speakers
and then i was going to put these through an amp, something like this (Tell me if it's suitable or not!)
Amp
And then the subs, i want something that can kill a man from 50yds away, so i was looking at these,
Subwoofer
OR this smaller version of the same thing
Subwoofer II
What are peoples views on subs with amplifiers built in? And are these actually any good?
Obviously i will need the new HU, but i can't find a cheapish one with more than one pre-amp output, help, anyone?

One thing that from all my trawling i still have no idea about is the wiring, i know that it's important, but what wires do what and how good will the wires that will need have to be?

Also, anything else i am missing?
Don't be afraid to be harsh, because i am totally new at all this, and it's the only way that i will learn.
Thanks

Edits: The reason i am buying things from car audio centre is that they have a shop near to me and i would like to buy some things from a retailer near me so i can talk to them face to face if anything goes wrong