This is right up my street. I've installed quite a few systems over the years, including a full custom install in my Mk4 R32, and it sounded fantastic.
The biggest piece of advice I can give you is don't get hung up on wattage. A lot of people see huge power numbers and assume it's going to sound amazing, but that's really not how car audio works.
Your first priority should be a good quality head unit. Personally I'd stick with Alpine, Pioneer or Kenwood. I'd avoid the cheap Android units unless you're spending decent money on one, as the audio quality is generally nowhere near as good.
If your budget allows it, I'd also strongly recommend running your front speakers from a dedicated 4-channel amplifier rather than relying on the head unit's built-in amp. In my R32 I ran Focal 165AS components front and rear from a Kenwood 4-channel amp, and the difference was night and day. The speakers played louder, cleaner and with far more control than they ever did from the head unit alone. It's difficult to describe until you've heard it.
As for your mate recommending a 1500W sub, 1000W amp and 700W speakers... I'd honestly ignore the wattage figures. They're largely meaningless unless you're talking about RMS power, impedance (ohms), sensitivity and matching the speakers to the amplifier correctly. A well-matched 300W RMS setup can sound miles better than a badly matched "1500W" setup.
The first thing I'd ask is:
* What's your budget?
* Do you want a stealth install or don't mind losing some boot space?
* Are you after sound quality with a bit of punch, or are you trying to shake the windows?
For reference, I'm now in my 40s so my tastes have changed a bit! In my Mk6 GTI I'm running:
* Alpine iLX-705D head unit
* Focal IFVW front components
* MTX Thunder TR8PT active sub
It's not earth-shattering, but it gives me lovely crisp sound with enough bass to put a smile on my face without rattling the whole neighbourhood.
I actually wrote a fairly detailed guide on choosing a head unit over on the Mk6 section if you're interested:
https://www.golfgtiforum.co.uk/index.php?topic=290235.0If you let us know your budget, I'm more than happy to recommend a complete setup that'll give you the best bang for your buck.
