Brad nailer or finish nailer for this wall?

An 18-gauge brad nailer can work for light boards if it accepts nails long enough to pass through the shiplap and drywall and still get a solid bite into the studs. Check the nailer’s capacity before buying, since some models take shorter nails than others. The total thickness matters more than the label on the tool.

Mark the studs before hanging the boards, and choose nail length with the board, drywall, and desired stud penetration in mind. Watch for wiring and plumbing locations when fastening. I wouldn’t count a nail that only goes into drywall as meaningful support.

If you want one nailer for a range of trim and wall projects, a 16-gauge finish nailer is a reasonable all-around pick. It gives a stronger hold than a brad, though the holes are more noticeable and need filling. For thin, light shiplap, that extra strength may not be necessary, so I’d decide based on what else you expect to use the tool for.

The board ends need backing too. Try to plan the layout so each end lands on a stud or add suitable backing where that isn’t possible. A few fasteners into drywall between studs don’t replace fastening into framing. Adhesive may be used as a supplement if it suits the wall and installation instructions, but I wouldn’t rely on it to make unsupported ends secure.