EDIT: Right now at July, 2020 o reach Level 5 on the garage you need to reach 300,000 points between all your owned cars and its upgrades (at the original trime I wrote this it was just 70,000). Every single one of them adds to the total count of the Garage Level.
I was able to reach it with 4 Class D cars, two of them heavily upgraded and just 2 Class C cars, one of them fully upgraded to parts Lvl 8, so it will be easy to reach to however plays actively the game, even with just the campaign.
The main purpose of the Level 5 means that better class blueprint cards are unlocked.
In Garage Level 5 are from these cars:
- Class B: Ferrari 488 GTB
- Class S: Lamborguini Centenario
- Class C: Pininfarina H2 Speed
- Class A: Dodge Viper GTS