[Ishj]

First off, a bit on me. I live in Mauritius and I am a student and quite an audiophile, and thus on a very strict budget. I listen to many genres of music, ranging from metal to rap to EDM to Jazz. The E90 currently has the base audio system, which I will be replacing with a Joying android head unit, made for the E90 generation.

Here is what I already have:
A 480W 4ch pioneer amp,
A 1000W 4ch pioneer amp,
The head unit.

Here is what I want to do:
A corner subwoofer in the trunk with a Fiberglass enclosure I will build myself(Budget).
Midbass woofers under the seats
MiniDSP 6x8 that I will tune myself using REW and my calibrated mic(unless there is a cheaper DSP)
The 480W amp bridged sending the two underseats 120W RMS each
The 1000W sending 60 RMS to the 2 front and rear speakers
Keep the rear OEM woofers and reduce the power to them for the time being(If possible)

What I need to buy:
A subwoofer
An amp
two underseats

I was looking at either the Image Dymanics ID12/ID10, and a Rockford Fosgate 250W amp, and Audison AP8 woofers.
I am not sure these are the best and cheapest options for me.
Here are my questions:
1. Can I keep the OEM 6.5" midbass till I save up for the 8"?
2. What is the volume of a typical corner enclosure, can I put the ID12 there instead of the ID10?
3. Will the sound get through to the cabin enough without a ski-pass?

My total budget, excluding the DSP that I will be getting later, is $550, and that is including shipping to Mauritius.