A great place to find load data.
Let's start off with Shotshell. I'll primarily be talking about target loads. This Video does a much better job showing you in 60 seconds than I can over text. The key part is 1:10-2:10.
Now this MEC 600 Jr is the exact thing I use, which was actually what my father used as a child and passed on to me. The video states that you can convert it from one gauge to another. While this is true, I don't think it's practical to continuously swap back and fourth. Also, Shotshells are about as easy as you can get. You're not exactly looking for precision when busting clays. The hardest part here will be finding affordable lead.
A word of caution, it's not always safe to swap components (ie, primers, shells, wads) without referring to the charts. This is obvious with gunpowder, but people don't always think about it with other components. Some primers are hotter than others, and some shell/wad combos might give you a tighter seal. Here's a couple examples below pulled from the Hodgdon sight above.
||Rem. TGT 12
As you can see, just swapping out your wad increased your pressure by 700 psi.
As for inspecting the shells, cracked plastic is a no go. So is misshaped metal base. Look inside as well. Bent plastic is fine though because these shells are quite flexible. Once you put in the shot and crimp it shut, it'll return to it's original shape. Sometimes you might find a fickle semiauto that doesn't always like to feed it, but I've had no issues with my pump, and certainly no issues with single shot and over/under.
It also doesn't need to be cleaned/lubed unless the outside is muddy. Rifle/pistol does.
Warning: Do not use any random shells. Most of those cheap shells are made to be disposable. Look inside and you'll often see a cardboard base. Just imagine this thing coming unglued and lodged in your barrel. Do so at your own risk!**
What do I reload? 12 gauge 2 3/4 Remington shells, 7/8th ounce shot, and WAA12L wads. Primers are whatever's cheapest with minimum acceptable powder. Also, I use reclaimed shot. Why? Because I'll use it for busting clays, and the cheaper I can reload, the more I can shoot. In reality, most powder/primer/wad combos are acceptable, but it's best to reference the load data before you drop $50 on primers.
Also, reclaimed shot? It's exactly what it sounds like. All these trap and skeet ranges shoot in the same general direction, right? They'll occasionally have a company come out and dig up all the lead BBs. This company will then clean them, remove any steel, and reapply graphite powder. The end product looks like this. Not perfectly round but it breaks clays just fine.
For entry level reloading, this is as safe and easy as you can get.