Unable to unlock Ultima RS

I collected all blueprints for the ultima rs in the special hunt and the only thing left is 4 epic parts. but i don't have enough credits to max it in order to get the key. currently, the car has all upgrades at stage 10 with no epic parts applied, but the next upgrades and the epic parts gonna cost hell a lot of credits (500k × 16) and i don't think i can grind so much, i can't play 24/7. is that bad? or will i be able to get the key someday in the future? i don't want to believe that i did all the grinding for nothing.