True about rustler and sparrow, I think the solution could be making them gain less than double hp
Late to the party however, I feel no reason not to repair your air vehicles and also no reason to not be able to buy a plane. (For a super steep cost this could be a very good way to help clean the economy).
The ability to buy planes is an upcoming feature. There are already large checkpoints at the 3 main airports that repair and refuel your aircraft. These checkpoints will be given additional options in the future.