Hello I'm new ^^

If you want to learn basics of vehicle creation, check this:
Introduction_to_Vehicle_Creation
Even if some parts of it are outdated, it is very good.

 

Hi and welcome sir

soo, to answer your question about "must have" mods, bascially anything that is in the top list in each category, or featured (there you'll mostly find up to date mods as well).
As for cars, there is quite some stuff to choose from. And maps ; My personal favorite is "Altitude", you should be able to find it pretty easily

but restricting to cars and maps isnt really needed here since there are a bunch of mods that add customization parts for existing vehicles for example.

Just check the repo and you should be able to find what you need quite fast