Here's my point. You are leasing the truck for 5 years. This means for all the payments you are making you don't own the truck after 5 years.
At the 5 year point if you buy it, you'll likely finance the 12k which most folks do. Most will finance that 12k for 4 or 5 years.
In five years will finally own a 5 year old truck and be making payments on it for a total of 9 or 10 years.
The estimated 12k will most likely be well more than the truck is worth at that time. Just hit http://www.kbb.com
and play with the numbers using your truck as a model on 5 year old trucks.
Most if not all of the mods you want to do will not fit on a new Ram that far down the road and you'll only be able to sell them for pennies on the dollar because you won't be able to offer them as new with warranty.