Personally, I'm a fan of JVC head units - they for some reason have a bit of a rep for being cheap, but I've used them for years in a number of different car audio builds and have never had a hiccup or been dissatisfied with the sound quality. Plus, they have a built-in Pandora app that allows you to change songs/stations from the radio while connected via Bluetooth. There are some other units out there with similar functionality but they're usually more expensive, which makes the JVCs a good value for me.
For the sake of time, I'm just going to post a couple of JVC radios. I'm also linking to Crutchfield because of the extra stuff they send; it's nice to get the antenna adapter, wiring harness, mounting kit, and installation directions all from one place and not have to hunt around for it. Their service has always been top-notch for me as well.
Pstall: How do your stock speakers sound? Most/all factory head units come with some baked-in EQ curves, so changing the head oftentimes makes the biggest different in the entire system. If your stock speakers aren't blown, you can spend a bit more money to get a radio with some crossovers and TA and may very well be happy with the sounds. Also, replacing stock speakers in stock locations negates a lot of the gain in a more expensive set of drivers, so I tend to put more money into a radio for this reason.
JVC unit with 3-band parametric EQ and crossovers.http://www.crutchfie...KDR80BT&tp=5684
JVC unit with 5-band EQ, better crossovers, and time alignment. Time alignment adjusts the speakers to make them sound like they're the same distance from you and makes a world of difference in sound quality and imaging. The TA in my head unit really made my stock Subaru POS speakers sound pretty good. IMO definitely worth the $50.http://www.crutchfie...684#details-tab
Biscuit: I did a buddy's stereo in his 03 Explorer a couple of years back and the radio is a piece of cake to swap out. The bezel around your center dash just pops off with a little bit of prying force and the radio comes out with a couple of screws... literally a 5 minute removal. Here's a YT video showing it in action: http://www.youtube.com/watch?v=MXs2JWFhUHQ
As far as radios go, I'll make the same recommendation to grab one with sound advanced features and see if you're happy with the stock speakers. For double-din, I am pleased as all get out with this one - I have had it in my car for over a year now and haven't regretted it for a moment. Crossovers, Pandora app control, time alignment, and 5-band EQ. If you go to the reviews, my review is actually 4th from the bottom on the last page (I'm Brian from Kansas City), and I still fully support those comments.http://www.crutchfie...dr#overview-tab