If he lets you shoot on the property, bring a tape measure and a few different targets/holders.
I have been patterning buckshot for a month or two now, and even with 3 target holders(set up at 5,10,and 15 yards); it takes time to shoot, run down range, reset, shoot again, ect...
several popular pattern distances tend to be: 5, 7, 10, 12, and 15 yards(or 15,21,30,36, and 45 Feet)
I find it very hard to find a distance farther than 15 yards that may be useful for HD type use(what I am mainly concerned about), but can understand longer distances for Hunting, LE, and Military use.
I would say for a class, Rio royal buck from somewhere like
https://www.ammosupplywarehouse.com/category/SGB0/12-GA-SHOTGUN-BUCKSHOT.html would work. Classes require quite a bit of ammo normally. premium ammo can and will be very expensive to buy in bulk. A lot of shotgun classes will have a segment where you pattern your buckshot to show how it performs at different distances. You might be able to bring a couple of boxes of a few different premium rounds as well (I would contact the instructor to be sure first).
For actual defensive use, you will have to define what distances you will need to shoot. Then you need to decide what size pattern is acceptable for your purpose. Once you have that figured out, you can then pattern several different rounds to find something that fits your criteria.
My criteria is a 5-8 inch pattern at 15 yards, which Rio royal buck 00 does consistently in my shotgun. YMMV
Hope this helps, and have fun shooting.