1st - your head unit has to stay. Its not really optional on this car.

2nd - The SS is a balanced head unit and basically no matter what you do you have to use balanced input/output DSP and amplifiers so cross the cheap stuff off you list.
Failing to do this will give you a nasty steady hiss and pops galore.
the factory head unit puts out a dead flat 4-5 volt preout signal so its glorious for good equipment down the signal path. This means that Alpine amp may need to go.

3rd - even with a proper amp/dsp combo, there will be some minor hiss and a turn off pop since there is no true remote power wire for the radio since every chime in the car runs through the headunit. I have the JL TWK 88 and I maybe trying a Masconi instead to see if that can cure any of the hiss/pop.

4th - the speaker placement, specifically the tweeters are god awful on this car. A speaker swap alone doesn't do much to help. You may find like most of us that you need to correct the tweeters angle.

5th - the trunk is HUGE. I have 3 12"s and I still don't block the spare.

For the speaker selection set a budget and go listen to some stuff. I fall into the trap of everything sounds better when you read about it but the reality speakers don't sound the same to everyone so use your ears not your eyes.
There are a lot of component sets out there for around $300 that do a great job and with all different sound characteristics so you have a lot of choice. After that $300-$350 mark; I feel you start getting less and less per dollar spent.

For your sub, if you liked it before, keep it. Also, take the 8s out of the car. its better to open up the trunk to vent more into the cabin for your new sub. You don't want those 8s basically being passive radiators and you definitely don't want them playing.