That frame, considering no rust, will probably outlive you and everyone you know so having it powder coated is not essential but you may want to if the current paint is badly chipped or scratched. Old Surly frames weren't famous for their paint durability but newer ones are factory powder coated and a lot better.
That said, a major upgrade is going to cost a large fraction of the cost of a new bike and a new Disc Trucker will have, current components, disc brakes, thru axles, clearance for larger tires, and be tubeless ready, all of which your present bike won't have.