20lbs is enough, not sure on water flow you should be good for a while with stock pump best advice is take it slow, are you purchasing cured live rock or uncured, try to get a pound of live rock per each gallon, i have 23lbs in my 24g aquapod and i still have plenty of room in my tank.
Check your water paremeters daily till your cycle is over, id check them daily after that anyways because its so addicting.
Make sure you get testing kits for (PH, Ammonia, Nitrites, Nitrates) those are your primary ones, later you can get (Calcium,alk,mag) also get a hydrometer.
get cured rock cycling uncured takes to long but i guess in this hobby patience is virtue. i am new to this but ill help as much as i can.