Purchasing First Firearm - Suggestions Welcome

[Justice]

A Maverick 88 is an exact copy of the Mossberg M500. Barrels and aftermarket stuff, of which there isn't a lot, will fit.
1. Depends on what you're doing. An 18.5" M88 isn't a 'duck gun' though. (They're really good upland bird and rabbit guns though.) Short barrels are great for those critters.) You need a 26" + barrel for ducks and geese, like Gunexpert007 says. You can buy one with changeable chokes that will give you a very versatile shotgun.
There is no 'minimum' quantity of ammo. That's more about your budget than anything else. You'll have to pattern whatever you do buy too. That's basically finding out where the thing shoots and how dense the pattern is with different shot. Easier to do than describe.
2. You should have been taught this on your PAL course. A trigger lock is the absolute minimum. Then put it in the back of a closet. Doesn't hurt to put a pad lock on the closet door as well. Storing in a case is not a good idea even in a silica cloth. Cases tend to attract moisture.
"...not that easy for a new shooter..." Isn't easy for anybody given all the government persecution.