The 250, 350, and 450 bodies are all the same, just make sure you enter the correct year. Although, 1999 - 2004 bodies and dash are the same. The dash changed in 2005.
The 450 is probably not listed because it is primarily a work truck only.
Your body style would be considered 'Pickup', because the cab does not have any kind of rear seats.
As far as what kind of stereo to buy...the sky's the limit! It all depends on what you want it to do and what capabilities it will have. Basically, there are three different categories which are: Basic stereo, which some have a CD player; stereo with DVD video player; stereo with DVD video player and GPS Navigation. Some of the more advance also have Bluetooth so they can connect to your cellphone or MP3 player, some have a Satellite Receiver built in, etc, etc.
Most will put out about 35 - 40 watts, which is enough to drive 4 speakers at a reasonable level, but if you want to rock out everybody around you while sitting at an intersection, you will need some power amps.
Some have output 'zones' which will allow you to run a second set of speakers, which you may want to do unless you're planning on a separate stereo for the rear.
My suggestion is, figure out how much you want to spend, look through Crutchfield's page and see what is available, check reviews and prices on Amazon if they carry it, maybe narrow it down some from there, make a written list of what features you want, then call Crutchfield and talk to a salesperson to get all of your questions answered.
Choosing a stereo is a matter of personal preference, so there is no way I can tell you what to buy. One piece of advice though, don't choose a stereo that is totally touch screen. Pick one that has manual basic controls on the side or bottom of the screen, such as on/off, volume, and any other thing you may need in a hurry without taking your eyes off of the road. The screens can be hard to see, especially when there is glare present, and most are menu driven which requires a serious distraction from driving to navigate.