I wouldn't even worry about the rear speakers. With not a whole lot of interior space to fill, I would just worry about a solid amped front stage and a sub. You'll be suprised how quickly you'll forget about rear speakers. Some amps like Kicker (shameless plug) feature a built in high input converter so they can be ran directly off the headunit. If you're looking to replace the front stage, it probably wouldn't be a good idea to use the factory tweeters as they'll probably start canceling out with whichever aftermarket ones you use.