Paint for garage question

Not exactly a bathroom or plumbing related question but I am remodeling my beat up garage. The previous owner beat it up good, 20 years of water stains, looks like he threw a hammer at the wall in one place and went crazy with a powerwasher with the pinpiont setting in another area. Full drywall covered and insulated, the area facing the house is mudded and taped per code. Its a shame... 3 car garage with hot/cold water, its own breaker box with 200 amp service, even a 220 outlet for whatever reason. I need to do it justice and fix it up proper. All of the damage is pretty much from the floor to 4' up so instead of patching and mudding I plan on lining the lower 4' of the garage with OSB board and painting the whole garage white. The OSB should make for a good shield and save me some drywall work at a the same time. Should I just use a decent exterior paint or prime it with Kilz or Zinsser BIN which is $$$. Assuming the exterior grade paints will block the stains and save me time and money?

Drywall is supposed to be installed across the studs anyway (the grain of the paper is stronger that way), and already 4' wide. I think it may be easier to just cut out the bottom 4' and replace it. Painted OSB will likely always look like painted OSB, and then you'd have the edge to deal with. It would be a shame to paint it all and then find out that the stains bleed through, so I'd opt for a stainblocker. You might get lucky without it.