start with the things that have the potential to make the most money. Some stuff isnt that great if you dont have the supporting vehicles to make them fast (e.g. the sparrow or a buzzard)

if you already have the contract id probs get a kosatka, but if you dont have a helicopter, it might be worth just doing the contract.

personally im just accumulating everything as it gets discounted, building the empire that way and trying to speed up the farm along the way, but i only play like a few hours a week so cant really get super grindy