[makkan00]

I will take the plunge.
We cannot confirm what head unit (HU) / sound system you have, unless you post photo of your HU and tell us how many speakers / drivers you have.

Most of people prefer OEM HU as that it very capable of doing most of the things and beauty of retaining it is, you do not get issues when modules of your car is being updated.

If you are very keen to have aftermarket HU, still you can retain your OEM HU in trunk. If you click search you will find posts how to do that.

About Technics harness, it will take signals from your HU to amplifer and back to HU from where it goes to wires of your drivers.
In between how many amplifiers do you want? It depends on you.
However most of people keep their subs separate to their front mids, and for that you will require additional subs cables (you can get that from Technics as well).

Hope that helps.