If you are a normal person, a demo car is fine. If you are into spic and span, you probably wont tolerate it. Mechanically, there should be zero difference between a demo and a new one.
Keep in mind there is no need for warming up, theres no overreving, no misshifting, no slipping of clutches, no riding the handbrakes, etc. Its harder to mechanically mistreat a Tesla outside of driving the battery down to empty.
Cosmetically I dont think the tens/hundreds of testers do anything untoward to the vehicle. Getting in and out of the vehicle does not accelerate wear. Running ones finger along the paint does not accelerate wear compared to ownership usage. You might have a few extra scuffs on the door sills or whatever, but it'll look like how my car looks today and I bought mine new (and I am not a showroom/spic and span owner)
What you do have to take into account: demo cars aren't cycled around. the ones in the showrooms here are earlier builds so you'll have a much earlier VIN, earlier official build date, and earlier quality (if you believe in that stuff)