We do a lot of work that we get through realtors on addendum's to the contract after a home inspection. Usually the people want it fixed less expensively but still decent looking.
We buy from either lowes or HD a pvc door jamb kit. Use a multi master and cut the jamb above the rot, high enough that your piece is at least 7 or 8 inches. On the hinge side we will often just right at the bottom of the hinge, helps hide the joint.
Then cut a piece off the jamb kit to fit the opening. It's already made up and the right size, etc., no need for custom modification except one thing. Where the threshold is sloped, you have to cut the fatter part of the jamb at an angle cause of the way the door is built. I use the multi master for that cut as well. Then install it. I use shims behind it and behind the remaining piece if needed which they usually are to get the joints lined up.
As far as the way the threshold attaches a lot of the time they are glued down somehow and seem ok. I will put a screw through it that is then covered by the jamb when it goes in. If both sides are rotted and the brick is as well, you can pull the threshold, shoot into the end of it again, and slide the whole assembly back in.
Hope that helps.