^
Great advice!
I shoot 3gun & IDPA (both pistol and PCC).
As ATLDave said, you'll get no benefit from going above 9mm, it will actually hurt you financially (brass, loading) and time wise. You don't get to pick up brass until your squad is done on that stage, and then you're on your way to the next one. At the end of the day, once the stages are broken down, you can police your brass. During the match, you're loading, on-deck, shooting, pasting tgts and picking up steel. Brass recovery isn't really an option unless you want to get known as "that guy".
Most new guys have issues with the stuff Dave mentioned, reloads, accuracy from non-standard shooting positions, and just doing stuff on the clock. The accuracy standards for equipment really aren't that high, your average pistol/carbine are easily good enough. What kills guys often is reliability (lack of) and not engaging targets at all. Sounds simple, but there's a lot of tgts to be engaged during a stage from multiple firing points.
We've got a couple guys in our club that shoot Kel Tecs and I haven't seen them have any issues. A blowback PCC is generally a pretty reliable gun as long as it's put together right. Longest shots, really depend on the club running the match. We normally have a "long" stage that goes out to 300+ yards. It sucks for PCCs, although some guys are quite good at that distance. Then the PCC guys have an advantage with being able to shoot pistol TGTs and some shotgun steel with their PCCs, whereas the 3Gun guys have to use a pistol & shotgun for more of the TGTs. Shotgun is harder to keep loaded, pistol isn't as accurate. Basically in 3Gun the PCC guys shoot a stage withing a stage that's modified for them.
My advice follows ATLDave's, go to a match, shoot, concentrate on safety and accuracy (speed will come) don't try to keep pace with the experienced shooters, expect to be humbled, learn and have fun.