Increase value = sell skyline and invest in a house.
most things you do to your car will always depreciate, unless you keep it mint and no one else has it. then it becomes a collectors item.
increase speed = cut wire 53. more money you have the faster you can make it go.
if your going to keep it for 6 months only, keep it stock as possible and keep the k's low for better resell value. if you want to sell it quickly then you have to sell it cheaply.