Do you already know the model number for the Kenwood aftermarket head unit you want?
If you do you can go here Find what fits your car - Crutchfield
, enter your Jimmy'* info, and see which head unit wiring harness, speaker harness, and steering wheel adapter you need.
As for bypassing the factory AMP, I would bypass it for sure, as when you get new speakers they are most likely going to be able to handle more power than the factory ones could, and if so the old factory AMP won't push them.
I'd try to get a head unit with 2 pre-amp outputs, and get 2 separate AMP'*, one for your door speakers, and one to drive your subwoofers.
Yes you could get a multi-channel AMP and run it all, but it is not going to sound as good.
With all of the harness'* you can get if you look, you should not end up having to cut any factory wiring at all.