A lathe is a major investment, but is critical if you do any amount of serious scratchbuilding. As a ship modeler (predominately), I'm always turning gun barrels and masts and other fittings like vents, capstans, etc...
A lathe is not the solution to all the problems however. It takes time and patience to learn to use one, especially if you're looking for consistency, say, like nine 16"/50cal. barrels for an Iowa-class battleship.
A punch and die set is a good investment, along with a nice reamer set and a PE bending brake, either Hold-and-Fold or Etch-Mate. You don't use them all the time, but when you do, you'll never figure out how you could work without them. It expedites your build, and improves the quality.
Jeff