Agreed. A cars value only decreases. It can increase if its really old, or rare. And v6 mustangs will never be rare as they make sooooo many of em.
If you dont want to hurt the value, only do mods that you can reverse. I personally only buy 100% stock vehicles. They are less likely to have ghetto work done on them and I have a clean slate to work from. There are a lot of people that feel that way.