My Advice,
always stick with what you know, you could spend 5k and have a worse basis than you currently have.
a few important things to do on her before its too late are waxoyl the underneath ( a rusting chasis was my mk3 downfall) keep your eye out for a good 2nd hand fuel tank, the metal collars that the cap screws into rust and become a pain in the arse MOT fail because they are pressed into the plastic the are not replacable, new tanks from vw are £200+.
source a good second hand passenger seat, eventually the boulster will wear in you drivers seat and the metal bars hurt your arse. If you have the seat foam its just a case of a swap, if you dont you might need a new cover because it doesnt take long for the fabric to tear.
My personal opinion with cars is, the makers spent millions designing and developing for someone to come along and change it is stupid however, make your car the best the designers intended eg change the wheels to BBS, put leather in, stainless system etc.
Sub and amps dont have to be chavvy, if you like music you will appreciate a good system.
Dont Sell her!