Keep the price lower? The factory stereo still being in the car will not lower the price of the aftermarket unit that you'll buy to replace it. Anyway, a wire harness adapter will still be needed to integrate the aftermarket unit into the factory wiring. Don't worry about the speakers, you don't have to replace them if you don't want to. The aftermarket stereo will have more than enough power for the stock speakers.
As for what unit, it up to you. What's your price range, what features(other than mp3) do you want, what brand do you like? After you've figured that out, shop around.
Here's a good place to start. You don't have to buy from them, but you can get an idea of what's out there. Click on the features you're looking for under "NARROW YOUR SEARCH"...
http://www.crutchfield.com/g_300/Car...s.html?tp=5684