Best Sim/Game for user-created cars

Wanting to find a semi-realistic driving sim that allows me to create my own car from files - not just paint/color imports - an entirely new car.  The Googles are not really helpful, although it looks like I have to do a PC game, since this isn't a viable option for consoles.   

Here's my biases:

GTAV allows me to import files of a sort, but not a game with legit tracks.

iRacing does skins, but no user-created "vehicles".

Perhaps an rFactor or Forza ish game - not sure how much creative importing can be done on those games.  

Inputs?

You can build them, or you can race them, but you can’t do both. Even if you find a game like that, it’ll be ridiculously cheesy on one end or the other, or both. 

Assetto Corsa seems to be the ticket for PC sims that allow for mods.

rFactor has tons of mods.

Assetto Corsa has some good ones I've heard...

https://www.youtube.com/watch?v=oOCHk3JoSps

If you do this, would you mind making a thread?  I'd like to learn how to do it.

rFactor if you have an older PC. 

Assetto Corsa if its a more recent PC. 

Both have a ridiculous amount of mods out already and a relatively active user base. Should not be hard to find resources/tools/tutorials for either. 

I have dabbled a little bit with parts swapping (wheels) on AC and used some software called 3DSimed.  You'll probably need something like 3DS Max or Blender if you intend to get really deep into modding and creating cars. 

Also for AC there is a program called Content Manager that has a lot of tools built in that can streamline the process of getting a new car into the game, creating AO maps, etc. 

I'd love to learn how to mod more, but dont have the free time. I can barely find time to simply play the game. 

I spend more time downloading and installing mods then I do actually playing. Haha. 

Also, I dont know of too many active forums for AC modding. There are tons of facebook groups though where files are shared and people communicate.  What types of cars and tracks are you into?

A lot of resources here if you are into drifting

https://www.acdriftingpro.com/

In reply to boaty mcfailface :

I was researching this last night, and I did find a lot of drifting setups.  I hadn't gotten too far into the actual modding details, but it seems like there is a fair amount of info on Asetto Corsa available for creating new cars, etc.  
 

I'm looking for the ability to add some oddball cars (like my Granada lol) and lesser-known but real tracks (Harris Hill, Driveway Austin) into the game.  Kind of a 24 Hours of Lemons sim thought process.

Back in the day when I was into racing sims, this was pretty cool:

http://www.racer.nl/

In reply to FIYAPOWA :

I dove in this weekend on learning the ropes of 3ds max, and kseditor.

I am working on importing a model from Forza 4 currently to follow along with the tutorial vids I am using. I can grab a link for the videos and share here in a little while.

The process would be maybe only a little different if you are importing a model from elsewhere without textures but this is an easy way to learn the process I am finding. 

Here's a link

Assetto Corsa Car Converting by Wack

I run rFactor (version 1) and the moderators of my group will do their own modelling.  We have run sprint cars to go-carts and right up to 2015 F1 cars.