If you're already thinking about selling it or upgrading it then there's no reason to mod; usually you should only mod a car if you plan on keeping it and definitely don't expect any kind of returns on that, you mod for no other reason than yourself. It certainly can last you another 8 years and still look good, but you should focus on paying it off.
Think about how nice to be able to say that the car is yours and that you aren't making payments on it? If you're concerned about spending money on maintenance remember that it won't be cheaper to just buy a new car. Too many people would rather spend $30,000+ instead of paying a $1,000 repair bill, it doesn't make any sense.