Re: First Jeep
#1 upgrade (and it's free) is the "Driver's Seat Insert" upgrade (see also: Steering Wheel Actuator).
Find someone who know how to wheel, "Tread Lightly", and practice trail etiquette and learn, learn, learn. Nothing changes a Jeep like the person driving it.
After that, like was said above, nothing's cheap on a Jeep. But what kind of mods do you want to do? Wheeling? Cosmetics? Streetability?