If you don't have a good size table saw to run it through, the best bet is a hand plane (powered might be rentable...). Secure the door on edge and vertical with cinder blocks or the like and draw guide lines along the edge to keep check on how you're doing as you plane. Finish up with 80 grit on a longish block of wood. "Break" the sharp edges with your sandpaper before painting.
Hello, This door already has a bore for a lockset.
No, the door is too flimsy.
I use mine as an exterior door on a storeroom. Mine has a roof installed over it. In any case be sure to seal the bottom and top with glue or caulking and keep it well painted. It won't last as long as an exterior door but you may get 5-10 years out of it. Again, keep the edges well sealed.
Unless the door is "prehung" then no as your existing hinge locations are unknown.
Sure, put it where you want it.
I think there is a less than 1" plug on all 4 sides/top/bottom, so take some off of each side if necessary, the handle side first. You need to leave enough for the hinges to grab
I don' t recall it being predrilled for a lockset