You know, it's time for a re-pipe in my opinion.
But if you want to stay with galvanized you can do the job, you just need 2 unions on the hot and cold. PEX requires a main manifold, something you don't have right now. If you want to introduce PEX, might as well do the entire house. You'll be glad you did.
Have you heard of copper type L? it is my first choice for re-pipe.