The Ford V10 has been suprisingly good and trouble free. The very early engines had some problems with piston slap and an exhaust resonance, but since 2000, the engine's reliability has been excellent. Just make sure the engine was well cared for. These engine's require very religious oil and filter changes to keep running well. If well taken care of, there is no reason for them not to run up to 250K miles without any major problems.
This engine is very powerful as is and I would leave it stock. It will pull just as hard as the powerstroke diesels, but with more instant response of a gas engine. To get much gain in a gas engine these days, you typically need to begin at the intake air assembly and exhaust systems first, then you can tune to that increased airflow. You'll notice most of the tuners for gas vehicles stress the horsepower gains and there's a reason for this. Both the Hypertech Powerprogrammer III for Ford and the DiabloSport Predator flash programmers will work on your Ford 6.8L V10 and are reasonably priced.