I am torn between two options:
1.) Getting a kicker amp (zx350.2 which should fit in the stock sub location) with speaker-level inputs and just using it to run my sub (Image Dynamics IDQ12 in 1cuft sealed). Then I would probably replace the front tweeters and woofers with something that could run on stock power.

2.) I have found a few small footprint 4 channel amps (ie Alpine PDX series) that I considered using to power some front speakers and then use the other 2 channels for the sub. Unfortunately, all of the amps that are small enough to be hidden in the BMW only accept RCA inputs. I assume here you would recommend a higher quality loc?

I am definitely not an audiophile, but I enjoy a nice sound and can tell that I dont get that from my current nonDSP system. My old vehicle was similar to option 2, I had a 4x100 amp running Boston Rally 6.5 components up front and a 10 sealed in back. Very good sound in a pickup truck.

I am considering the new Phoenix Gold RSD 6" components as they should fit the roughly 6" woofer opening, but I don't know about the tweeters or if they would sound good w/o some extra amp power.