This is exactly what I have done. I have one series with the Spark and Eve F3s and another that uses the 2013 mod from Frenky. In both cases, I created a RaceEvent where I chose my custom cars and the tracks that I would be running.
Here is a quick summary runthrough:
Chose one of the Marussia cars as a master
Create all your custom skins in Photoshop, etc.
In RF2, select the Marussia master and go into Tuning to put you in the garage.
Create a directory. (This creates a directory for this car) The directory will be located in rFactor2/UserData/Player/Settings/
From your desktop, move your skins into that directory.
Back in RF2 in the garage, click the arrows next to Variants and scroll through the skins to find the one you want.
Click the button to create a new driver. Name the driver, team, etc.
Do this for all your skins. You can also edit the AI in the garage if you want.
You now have two options. You can create a RaceEvent that will always include those drivers and tracks or you can just do it on the fly each time to start RF2. RaceEvent creation is already documented in several places. If you do it without a race event, start RF2 and select your car. Click Race. After the track loads, go to the Standings button and click that. Click Add AI and choose the cars you want to add. The master car will always be included, so you can click that one in the Standings list and click Boot to delete him from the race.