All are good options. In case you're wondering, in the beginning the drug dealers (at least this one I know of) are all in a gray hoody. Shoot them in the head preferably from behind because they will shoot back.
As for the Casino, save in your Casino and go play Roulette. Bet on Black or Red, it's a 50-50 chance and saving in the same casino makes it easy to reload when you lose, and save when you win.
best if you search ur face off to find drug dealers, get 10,000 dollars or any other way like the courier mission for the store near idle wood i think. then when you have 10,000 get an extra 1,000 and buy the house close to the sub-urban near the ese's gang area go in, save, and then ride to the betting parlor at the place where you and rider split up from sweet just check a map or something cause it doesnt show up on your radar but go there and bet on the one with the most odds like 1/12 or 1/11 the if you win go save and repeat, if you lose then reload but bet all. it will get extremly annoying trust me just stick with it you can do this right when you start the game.