Nothing. The mods add not a dime. That's the problem with modding the car. It's money gone.
Unless you really want a modified car, stay away. Most hot-rodded cars are driven very hard.
Highly modified cars can actually lose value in relation to unmodified versions. But, Dinan is a respect parts supplier, so the car won't take a big nose-dive thanks to that.
Don't put stock in the trunk decal, anyone can install one.
If the owner is pricing the car at a premium, I'd just buy a Certified car instead. Those mods don't add a ton of performance. Unless you're hung up on the automatic, the 6 speed is preferable.