Get a good old school carpenter. You have more damage than you are seeing. Living in New England we have lots of old post and beam homes to fix up. This unfortunately is not a DIY project , as I have repaired many including my own 1820's home. You need to remove any old siding in the way of the work to begin with, and see how much more is damaged. Then patch it back later if you are going to cover it up with new siding. This always opens a can of worms, and you may find more than you want to. Just remember everything can be fixed, just how much is the issue. Maybe the new siding can hold off for now while you fix all that structural damage.
The vertical posts rotting out is a big problem, and the foundation underneath needs to be solid.
A post and beam structure is totally different from a stick framed building, good luck in your search for a local carpenter, and ask for references.
Just getting you ready for the reality of old homes, sorry if it is on the scary side or does not answer your question on how to fix it. The only way to do it right is to change the entire beams.