Welcome fellow Texan!
There are 3 base models from Ford:
The standard V6
The GT (260 Horsepower Single Over Head Cam V8)
The Cobra (320 Horsepower Dual Over Head Cam V8)
Saleen is what we call a "tuner" company.
They purchase Mustang GTs and Cobras from Ford, give them a whole bunch of goodies, (Handling Packages, Superchargers) and sell them to the public...
Which is the best to get is determined what you want to use the car for and how much you want to spend....
If you want some street muscle to tote your groceries in while maintaining the option of adding modifications (or mods) to increase performance, the GT is a great buy at around 20-25k new...
If you want to make a statement with a little more horsepower and refinement, jump up to the Cobra starting at a little over 30k
and if you've got 40-50k to put down to annihilate any competition, a supercharged Saleen would fit the bill nicely...
As far as learning about Mustangs and parts, you've come to the right place...
Just ask away!!!
Welcome aboard