ok so what are you looking for in an HU? I dont mean general things that are somewhat necessary.
We can go with 6x9's in the back, but for the front I'd strongly recomment 6 1/2's. Simply to get a more realistic/pure sound.
For subs you should consider 12's or 15's. Being realistic I'd say 12's. As for power, it depends, you could go around the 500-600w rms each or 1000-1200w rms each.
Thats your call.
Note that 3 x 500w subs in your hatch will pound like the devil is trying to escape.
Depending on subs you'll need a suitable amp.
If you do decide to go with 3 x 1000w subs,(with suitable amp) then the 140amp alternator will more than likely not be enough. Even with an amp that powers 3 x 500w subs, you'll be cutting it close with that alternator.
Then you need an interior amp, which will depend on cabin speakers, which depend on your subwoofer choice.
So first you need to pick out a sub and HU.