It would be best if each speaker received its own channel. So a 4 channel would suit you best. The reason for this is because each side in wiring gets its own. A seperate signal is run to the front left, one to the front right, one to the rear left, and one to the rear right. With components, you'd run the signal from the deck, to the amp, and from the amp to the crossover, which will then run the signal to the tweeters and the mids.
If you're planning on an alpine amp, these should suit your needs. The first one is a bit pricier than the second..
The Alpine MRV-F340, then the Alpine MRP-F240. Specs could be found on their website,
www.alpine-usa.com
There are other amps out there though. But it depends on your budget. Also, dependent on your budget would be if you want to go with Kappas and an amp or References.
As far as passenger seat fit, that is dependent. It would be best if test-fitted.