Alas, there are no repair kits for vehicles that I know of. I agree, the garage repair fees are ridiculously high. A speeder about 2/3 or 3/4 down in 'health' costs like 11k+ credits to repair. I could be wrong, but didn't repairs cost around 5k in the original pre-CU? I suspect player cities will be planting garages ASAP to make some serious credits.
The quality of the resources definitely counts towards the durability of the vehicle, so the better quality, the longer time between repairs. I believe shock resistance is the important stat that will determine the hit points your vehicle will have. Of course, you can just get a crate of disposable (made with grind resources) bikes which would be cheaper in the long run than the costly repairs. This was a common trend back in the old days when aggressive critters/NPC's could easily take out your vehicle.