Hey guys, I'd like to ask some advice here. I'm really not an expert in how sound system works in car.

I'm driving a Saga SV with stock HU. I'm thinking of changing the HU since it doesn't have USB port to play Mp3. So,

1) What model/brand should I choose? My budget is around RM500 and below. Because my concern is only on USB Mp3 port.

2) Does HU differs in sizes? I mean if I choose any brand/model, will it fit to the car's HU compartment?

2) If I want to upgrade the speaker system, what item should I install in order to do this? As far as I know is the amp, speaker, maybe a sub-woofer. What spec is good for entry-level system?

3) Let's say I've bought a new HU (online), how much the is the installation cost in workshop?

Thanks in advance.