6.5" and 5.25" are not "drop-ins" - but it's very easy to make custom mounting baffles to mount them in the existing locations. Use 1/2" MDF - you'll probably need to remove some material from the door panel to gain full clearance - but other than that, go for it.
IMHO, I'd put one "really nice" set of 6.5" components up front and not bother with the rears.
My personal favorites are Focal's (JMLab) - 165KP and 165K2P. They're not cheap, so hold your breath when you negotiate the price with your dealer. You should be able to pick them up for around $425 (KP) - $525 (K2P) per set. You only need one set...