Air is more akin to actual rowing. If you're just doing it for cardio, get whatever's in your price range.
Rowing Machines
Joshua Wright
Daniel Lewis
It's not difficult once you get the hang of it, but if you continue to row over an extended period of time with poor form(especially using high resistance) it will fuck up your lower back.
A good general rule of thumb is using 70% legs, 20% back, 10% arms. The back and arms should essentially be used to drag out the row and maximize the distance rowed, while the power should all be coming from the legs.
At the catch(beginning of the row) you push legs, then pull with back, then pull bis until the handle is basically at your sternum, then you go exactly backwards, extend arms, bring torso forward, return legs to the catch position. If you can do that all in order and fluidly you'll be fine.
The biggest thing to avoid is being a sperglord who sets the resistance to 10 and throws his whole back into it.