Car stereos (the industry term is “head unit” for the piece you’re talking about) are pretty standardized, so they basically come in two sizes, one double the height of the other. So it’s usually fairly obvious whether one will fit in your car or not.
There are a number of companies that make a “kit and harness” for your car, usually you can buy them together. The kit is the plastic that goes around the edge, along with any mounting hardware you need, the harness is the wiring that goes in the back and gives you a plug to tie in to the rest of the wiring in your car.
If you’re fairly handy and don’t mind taking your time, you can definitely do the install, online retailers like Crutchfield provide some excellent installation guides that get you the basics, and you can transfer those to the specifics of your car pretty easily.
Also, you might want to go just drop by a car stereo place and take a look, see what they offer in terms of installation. Some places end up doing a basic deck install for like $15 when you buy a stereo there and they can tell you up front things like what kind of controls each has for the ipod, how well it’s integrated, things like that. Some of the implementations out there for ipod controls are just bad, but disable the controls on the actual ipod so you’re stuck with them. If nothing else you get to play with the knobs and see what you like ;)