Do not buy a truck until you know what 5er you want.
A truck limits your options. Even a 1 ton has limitations. You may decide you want a semi-tractor to pull some of the biggest models of 5er and there are good arguments for getting a semi-tractor even if you are not pulling a big, heavy, 5er. Check out the Escapees Forum for Heavy Duty Trucks.
linkA good used semi is cheaper than a new heavy duty pick-up and will have more miles left on the drive train than a new pick-up. They generally have a better turning radius and are built to last, but if something does break,... it is expensive to fix ;)