99% of cars are capable of being trackdayed (daied? day'd?) with no modifications at all that would affect the simulator - simply using decent brake fluid (e.g. AP600), some slightly better road legal pads (e.g. Mintex 1144) and, err, that's about it.
If they were being raced - this is the level LFS would be aiming at - in a low cost race series, then the addition of a roll cage, mild updates to ride height and spring stiffness (and, maybe, damping), and an extinguisher system are almost all that would be needed.
However, whilst LFS is planning on supporting 'realistic' setup changes/limitations, the other goal with respect to the road cars, is to give them a road car look and feel. So the 'requirements' of roll cages, extinguishers in reality are ignored, so that the cars are in road spec. One of the nice things about computer games is that alternative realities can be created - this is one where unmodified cars are allowed to race (other than the brake upgrades).
In short - we'll get your 'entry level track car' in terms of eligability and reliability, but 'standard road car' in terms of looks and feel.