Just Cause 2 DLC worth it?

dunno if u need weapons and cars sure why not

Yes, there is more fun with DLC.

beat the game first then buy the dlc and play again for 2 different playthroughs

The air gun is very fun.

Buy it!!!!

They are not really worth it. You can only buy the items at the black market, which is annoying because it takes time (you can only buy one item at a time if I recall correctly), and everything is very expensive so you may run out of money quickly. Moreover, when you die you have to buy everything again.
However I think there are mods which fix some of the problems. At least I know there is a mod to have the thrusters always equipped for instance.