A JL 4 channel amp, with two bridged to run the sub would do nicely. Run the rears off the head unit.
I would also leave 6.5" speakers. It is tough as 4 or 5.25" speakers do better with vocals, whereas the 6.5" deals with midrange / midbass much better. So I wouls stick with a strong vocal 6.5" speaker. Rainbows, Hertz and JL's would be on my list for that