Nothing wrong with a pump at all, at least to get your feet wet.
Nordic Components makes the best (and for almost all models) extension tubes and springs and that is what you should look at. At least 9 rounds, 10 is what I prefer.
As for loading gear, Carbon Arms (yes, it is my company) and Invictus Practical are the two best systems on the market. There are some differences between them, but you will find the majority of the serious 3Gunners using one or the other unless they are sponsored by another maker.
As for the autoloading shotguns, there are currently 6 solid proven choices:
VersaMax: Heaviest, softest recoiling, handles the widest array of shells. Needs some work and some parts changed out. Some need more than others. ARGO gas system like Benelli M4, but a little better design. There are versions, and in my opinion, even though Remington copied my personal VM for the Competition model, they did not go far enough and mis-stepped a few of my design features. I think most are going to be better off with a standard tactical and getting the mods done. The adjustability in the stock copies the M2 and it works well.
Stoeger M3000 (M3K and P3K): uses an Inertia system like the M2. Needs a little work unless you get a M3k or P3K version. A tad lighter than the VM. Very solid and reliable shotgun. Due to the hinged shell latch, it is the easiest to load and requires no tuning. The M3K is really close and the least expensive to get in the game. The P3K is from MOA precision and is ready to rock out of the gate. It balances a little more forward than the others, but the empty stock makes it great for shortening for the smaller shooters. That is what my 14 year old shoots.
Berretta 1301C: A gas piston system, but releases the shell on the trigger pull (Necessary for higher capacity reliability). Very good shotgun, however the controls are not "normal." It has the sharpest recoil of the bunch. Of the 6, it is my last choice, but others love it.
Predator Tactical P12: A very intriguing shotgun that uses the M3000 receiver and the 1301 operating system. I have one of the first ones in country and have been beating on it. Benelli controls, machined for loading from the factory, really ready out of the box. Less recoil than the 1301C.
Browning A5 (new model): Another Inertia gun with an auto-load feature that some think is necessary, but I am not in that crowd. A very good shotgun with a very loyal following.
Benelli M2: Still the dominant shotgun in 3Gun. Very reliable, the lightest and quickest. The 3Gun version is okay, but a LOT of $ and you can do better buying a field version and doing, or getting the mods done. The adjustability of the stock is a huge benefit. This is the shotgun I shoot, and prefer, most of the time.
Don't even think about a 930, 1100 or an SLP. They do not release the shell on the trigger pull and they are harder to maintain with a much smaller operating window of reliability.
There are a few others that are looking decent, but time will tell if they hold up. The Affinity is one, and the other is still in prototype and I am just about done with it and will be sending it back to the manufacturer with my final design inputs on it in a few weeks.