Be mindful of diminishing returns. Mods, if purchased used, can be sold without a loss once removed from the car.

What I'm getting at is I wouldn't spend money on improving a car you think you want to get rid of unless it's on parts you can resell or your adding to the value. Otherwise your throwing your money away if you decide a few months down the road your unhappy and ready to move on. Moreover, some of what you think might be improvement could actually turn off a potential buyer, as we all have different taste.

In summary, when you grow tired of a car it's time to sell. Plain and simple.