If your base weight is 10 to 12 lbs, and you're fairly careful with your pack, and this is a first DIY pack that you're likely to replace in a couple years then you don't need reinforcing on the bottom. If you're dragging it on the ground then maybe reinforced bottom would be good, the same 1.9 ounce ripstop would work. If you used silnylon or Cuben then you'de probably need reinforcement.
No need for grommet, just fold at the lower corners of pocket to make small holes for drainage. Or just poke a hole with hot object like soldering iron which will seal hole. Or just poke hole and don't worry about it.
I'm a minority of about one here, but why put pockets on the outside? Extra hassle. More places for water to enter or to rip. If you put your water bottle inside pack it will stay cool (or unfrozen in cold weather). I believe some mountaineers or Europeans are more into no external pockets.