For the standard course, you’ll need 195 rounds if you never miss. You’ll miss, so take an extra 50 rounds or more. 500 rounds will be plenty.
You don’t need a “duty” belt, just something that will keep your gun in the same place all the time. If your cheapo holster has a retaining strap, secure it out of the way.
For a hi-cap like your Glock, 3 or 4 mags should be OK. You really don’t want to run out of loaded mags and try to reload one while the clock’s running.
You don’t need mag pouches unless you’re shooting in an IDPA class. Most ranges give you a place to lay your mags. Just be real sure you keep your gun pointed DOWNRANGE while you’re reloading.
Remember that every shooter watching you has been where you are. Shoot safe, not fast.