You don't need to build the car to use Underwood's CVPI, if you look at my livery filedump I just posted, you'll see a CVPI that I built, but uses Underwood's base. The majority of newer models use Underwoods, Hurk's was great when it was released, but that was almost 6 years ago, Underwood's is almost the same age, but it is better in nearly every aspect. If you have any other questions or need help, feel free to message me on discord - covenetti_rs#0317
This isn't a bad start for someone who is new to texturing. Just for future reference, the person who's car you used isn't the person who made the base itself. For this CVPI, Chris was the builder who made it into a functional police car, Thehurk is the developer who made the base model.* So any car that uses hurk's base can use this livery. Same goes for your FPIU livery, that car was built by KIKI, but the base was made by GeorgieMoon.
* - I suggest using a CVPI with the base made by Underwood for next time, it's a newer, higher quality base that is more frequently used.