So I heard recently that it was possible to use TMUF cars in TM2. If I understood this correctly, the question now becomes, is it only the model/skin, or is it also the physics of the car (for example, TM2 and TMUF stadium cars behave slightly differently in my experience)?

As I currently understand it, this depends on how you implement it. The more general way (such that players don't need to have additional files ready beforehand) is with the vehicle archetypes. They use the ValleyCar model with CanyonCar sounds for whatever reason, but other than that they behave exactly like the TMU vehicles they're based off of.

The alternative is by using vehicle items. This lets you set things like the model and sounds, but the player needs to have the item locally before they can play the map. This is best reserved for titlepacks (see TMOne Speed for a great example), since those can give you the items.

I haven't messed with this much, so I apologize if any of this is wrong.

Thanks for your answer I guess I will give it a try and see how it goes ^^