Do you mind if I ask why you want to amp the speakers so badly? Maybe a better question to ask is what do you want to improve about what you have? S11eps is spot on with everything he has said, the only addition I would make is think about upgrading your speaker cables if funds allow. Putting in the filter capacitors is a piece of cake despite how complicated it sounds 
Personally the way I'd go about it would be to get the sub setup the way you like it first and retro fit the filters after. I say this because the frequency you cut off your existing speakers at is generally the frequency the sub comes in at. Also you'll only have to filter the bass speakers you have, Electrical block connectors shouldn't affect the sound noticeably either if you're afraid of soldering. The hu will allow some control over filtering but not as effectively as capacitors and definitely not as cheap - I wouldn't have thought they'd cost anywhere close to £20.
Definitely fit filters after adding the sub - otherwise you'll have a big hole in your sound until you get the sub up and running. Plus, you can't know what frequency you want your high-pass working at until your sub is in there.
The head unit uses signal manipulation to do the same job as the cap/resistor, and they aren't as accurate or steep cut offs as hard wiring the components into the signal line, however, they are free if you're putting a new head unit in and I personally wouldn't bother installing high pass filters if my headunit could do the job at 95% efficiency.
I only said £20 because, although an audio shop paid would probably less than £5, they're still going to want to make a profit - and generally the higher precentage margins are found on the cheapest stuff....
The speaker cable in the mk 4 will be ok until the point at which you upgrade the speakers or add an amplifier - it's actually fairly decent, and the time and effort I made in changing mine when I had a high powered head unit driving the speakes was wasted tbh, as a difference wasn't noticable - I'd save that for when you amplify, as you'll have to run new cables from the amplifier location anyway.
PS, As well as a High-end Kenwood head unit, I also have a rockford box with 2 x 12" punch subs in it for sale...
