If you are upgrading the front, get components rather than coax speakers. 6-1/2 and 6-3/4 are both fine. You'll need some kind of bracket or spacer for the door speakers because there isn't much space inside. You can fabricate the bracket from wood or MDF if you want.
You need to be more specific, as in actual numbers, when you state: "Not looking to spend too much."
Something like this would be decent, but it's only rated for 50W RMS. You could safely put more than that into it, but just make sure you set the gains correctly.
http://www.sonicelectronix.com/item_...TS-D1720C.html