Ca12bon

Hi guys,
I'm looking to upgrade my base speakers in my GS350. Please note that I am trying to save up cash for my emmigration this (or next) year, so I need to spend as little as possible. I'm looking to spend around $300-$500 in used parts, and then another $100-$200 for installation (installation prices are pretty cheap in my country)

Anyways, I'm a car audio noob, and I'm not sure what size / ohm speakers, or what type to get (coaxials, components?) I've read through all the speaker upgrade threads in this forum and also researched car audio upgrade videos on youtube, but I'm still pretty lost.

Here are my questions:
1) What speakers are the best bang for the buck up front?
2) What size speakers fit in the front? I see there's a speaker in the door, as well as the front dash
3) What ohm speakers do I need to get, or does it not matter?
4) I'm thinking of connecting 6x9 in the doors (I've read somewhere that 6x9's fit the GS350 door panels without modification), and 4" speakers on the front of the dash. Would $100 JBL 6x9 coaxials do the trick?
5) For the amplifer and subwoofer, what are some cheaper brands that perform well? And by "perform", I mean sound quality wise, since that's the most important to me

On my home PC, I went from using Logitech 2.1 channel speakers to an Audio Interface + JBL LSR 305's, it blew my mind away. I'm basically looking to do the same thing with my car audio.
Thanks in advance,
JC