One idea I have for this is sort of from the Battlefield Series. In it, the engineer can only repair a vehicle for a certain amount of time, if he goes over that time, his repair tool overheats, so then he has to wait a bit for it to cool down to start repair again. Another thing to add should be that only 2 repairs can be done at a time. Say 2 engineers are repairing a tank, and you come over and start repairmg the tank. Because you're the third guy repairing it, your repairs won't stack with their repairs, meaning you're just wasting your time. That way, repairs can't be op, and you could also add a script for AI like 'if two AI are repairing the same tank, another won't try to repair it too.' That way, repairs will be done by AI, but more in likely only about 1 AI will be repairing a tank, the possibility of a second repairing it will be smaller. I hope this helps!

or it could be consumable like a medikit^

Nah, but a medkit makes sense of being consumed because it used on a person. A repair tool just loses fuel or some stuff. Maybe making it so the repair tool has 100 ammo, and it is completely used if you repair 100 points of damage. So if an APC is damage and is at 70%, and you use the repair tool to repair it to full 100%, then your repair tool will only have 70 ammo left. That way, it can be used frequently for small repairs, and still be used for big repairs.

^I think this is the main detail of this thread at this point. I don't think they'll be adding a repair option before final release. But the above info is relevant, and sounds like could be put to use by a crafty modder.

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it should repair hitpoints (maybe 100 hp?) instead of % that way a tank will take longer to repair than a humvee and be balanced

Also it should be consumed at 0 if we agree on that