The thinking involved in driving a manual is very minimal once you get used to it, so I reckon any safety issues caused by that would be outweighed by a reduction in the unfortunately common situation of unintended acceleration crashes. You are lot less likely to drive through the nearest wall (or kids) if your instinctual reaction to moving when you should not be is to also go for the clutch and cut power instead of just pressing harder on the wrong pedal.
As in plasterboard sheets? I don't see why not if hand loading, plenty of vans will fit a 2400x1200 sheet (my Transporter fitted a bunch of plywood with room to spare). Loading one with a forklift is harder due to no side access long enough to fit 2400mm but that's a problem shared with tub back utes. If however your plasterboard pallet is side accessible a van with barn doors (like you'd buy if pallets were a priority) will allow you load it in fine.