I built for the KOH style Ultra4 stuff and have to say, it was eye opening.
To be capible, not even competitive, you will be surprised how much $$$ it will cost you.
Yes anyone can pull glass, install cage, throw nets and a cell in it and hope for the best. But you are going to loose respect from many on race day, and maybe even quit after the first race, due to motor blowing, rolling hard/bad, hurting yourself and co-dawg, etc. IF you dont take the time and funds needed to make a reliable, safe, and semi competitive rig.
IF I had to guess on the $ I have out of pocket on my rig I would say (Daym I hope the wife doesnt see this) I have about $12K in to my build. Then add that to the cost of the Freebie sponsored, 6 tires (2400$), 6 rims (1500$), Drivetrain items and front Currie RJD60 housing $9k atleast), Rock Krawler susp setup ($2k) Seats and harnesses (800$), etc, etc, etc...
As well as the 7 months of every available hour (within some reason) put in to the Jeep.
And we still lost a motor in the first 14miles lol.
I would Build a PreRunner to start, BUT Follow the rules from a class/sanction that you are interested in during the build on Anything hard mounted or big purchase. That way you can enjoy it to start and if at anytime you feel your tapped out on cash, you still have something to enjoy.
Or as grimm says, unless you crap cash. In that case, have at it
Oh or unless you own your home and can take a 2nd out on it.