My priorities would be to make sure the gun fits and make sure the gun runs. I'd worry about sights and capacity after that.
-Fit: Many shooters find the LOP on standard factory stocks too long. A lot of this is preference - though a smaller statured shooter, running a pump gun in a class that teaches a squared-up fighting stance is going to need a short stock. If your stock is too long, I'd look at a youth stock with something like a 12-13" LOP - this solution works for a lot of folks.
-Reliability: You'll probably be shooting a mix of bird, buck, and slugs through the gun in class. I'd run some of each through the gun in advance to try to identify any compatibility issues. In particular, I'd try to get a good volume of the specific type of birdshot you intend to use in the class through the gun in advance. Spend an afternoon running several rounds of clays, or just plinking. Try to get the gun hot and dirty. If it starts to choke on lighter target loads, consider punchier game loads (or just know that you'll have to do a quick strip/lube during a break after ~xxx rounds).
-RS barrel: Preference issue - you'll see beads, ghost rings, rifle sights and optics on shotguns at classes. I prefer some sort of sights for a couple reasons. First, when shooting buckshot at close distances (particularly, the ultra-tight-patterning low recoil buck common today), the "patterning" characteristic of the platform is removed, and you are essentially shooting a single projectile. Placing that single projectile with repeatable precision, for me, requires sights. It is extremely easy to miss small targets at close range with a load of Hornady TAP. It is also extremely amusing to the rest of the class
. Also, sights help to score hits when shooting from unconventional positions. You may not be able to get a solid cheekweld when shooting supine, but you can still align a set of sights. Finally, most shotgun classes include a block of instruction on shooting slugs at distance. Most shooters will find this section far less frustrating with a set of sights on the gun. I'd note, however, that the best slug shooter at one class I attended was a avid deer hunter running a bead-sighted 500. He was effortlessly printing groups at 75 yards that were half the size of the rest of us with rifle sights, ghost rings, and EOTechs.
-Extended Mag: Again, preference. Bigger mag capacity will likely seem like an advantage in some drills, but may seem like a disadvantage after a day of shouldering the extra weight at the front of the gun. No matter how big the tube is, it won't be big enough - part of the point is to spend the weekend stuffing shells. I'd configure the gun however you intend to have it configured and learn to work with it.
