Here is a thread on what to look for when buying one. It is not really for if one has been sitting, but you can take a look just the same. Maybe it will give ya some good ideas. The biggest thing I think would be get all the fluids changed. Make sure it has good fuel. And take it for a drive and see how it feels. That looks like a great truck though.
What to look for when buying a USED 99-03 PSD