You can use any head unit you want. I suggest a single din unit and a kit. Cheaper than double din, which you have, for the same features, looks better IMO and is easier to move to another car allowing you to take it with you or sell it easily.
Get a kit to fit it in the dash. I used a generic din pocket kit from metra. This gives you a pocket in the dash that holds CDs.
Get a plug so you can plug into the car's wires instead of cutting a splicing. Both kit and plug should be available at any decent car sterio shop.
Front will fit 5 1/4" pretty easily.
Rear 6x9"
You may also want an amp.
I like Alpine and Pioneer decks. Check
http://www.pricegrabber.com for prices. If you go pioneer, get at least a 4400 as it is flexible with what you can do.
Speakers I like Boston Acoustics Rx series.
http://www.tms-electonics.com has them pretty cheap and you can find more info on them at
http://www.bostonacoustics.com You would want the RX57 and RX97.
Not a hard install to do.