Hi, @Redin,
this is EXACTLY where I was with few of my friends almost 2 years ago. We have been annoyed by GTA public servers and I’ve decided to take us to FiveM.
I’ve made a purely free roam racing server with some custom cars and it was a blast! Then we wanted to expand it, so I was adding more and more cars. Later, I’ve figured out how to enable tuning on all of them (that support any). I had to do our own Drift script as well, since I couldn’t find any (at least no publicly) and all those little details, that make that beautiful result that my community wanted so bad for so long…
150 (up to 500 planned) cars, semi realistic handling for all of them, whole Nordschleife of Nurburgring… And we decided to go head on for RP lately as well, so full economy system, events, custom businesses, racing clubs, you name it…
But I will stay honest with you, it can be proper pain sometimes. If you really want the very best GTA engine and FiveM can offer for exactly what you’ve described here, you will have to invest hundreds of hours just in the development. It worth it for me, but if someone would tell me back then how much time I will have to put in this, I would probably reconsidered the whole thing 
Especially, when I had 0 prior development/coding experience. And it’s not just about getting in resources, you have to think ahead, defend the server against hackers and trolls, optimize things that other just didn’t etc…
Stop by sometimes, I’ll bet you will love it from what I read here 