Walls – Old structure: 2X4 studs at 24” on center

In order:

a) You can do it! You didn't mention anything about trying to move the existing structure, so this is based on my understanding that you are simply adding the tie back in to prevent further spreading. Personally I would use something more like a 2x8 or 2x10; a 20 ft. span is a long way for a 2x4, and you'll likely see some sag. I would put 5 nails into each rafter, in a rough square pattern with one in the center. But, you'll want to offset the nails a bit from each other so that they aren't going through the same grain (which can make it easier for a crack to split the wood where the nails are). You can cut the ends at the same angle as the rafters to get a longer surface to nail through.

b) If you do end up hiring someone else, a handyman could certainly handle it.

c) From your pictures, one tie every other is fine: the plates of the walls aren't going to move with a tie every 4 feet, and additional ones aren't going to do much.

If this wall is parallel to the trusses, it is probably not load bearing. At least that would be typical.

Water infiltration

You must address the cause of the water infiltration, or this will only happen again.

You noted the lack of housewrap, and that may be contributing, but since its by the window I'd say correct window flashing is essential. Check with the window manufacturer for instructions (they are all similar but for example: Andersen). The details really count, especially how to lap one layer over another, etc.

If you can do it from the exterior, I'd also air-seal the window to the framing while you have the chance. (e.g., Canned spray foam - low expansion).

You should add housewrap (or tar paper) wherever sheathing is exposed, and if you ever re-side tie into that later.

Framing

This is a reasonable repair to do yourself. It sounds like the rotted studs are possibly still adequate. i.e., if they are solid the remaining thickness and no water has reached the interior, you might be able to just live with them as long as the wall is closed up correctly.

You could also sister new wood side-by-side with the existing which would avoid tearing out material and damaging the interior finish. The key thing is that the header is still supported by the left stud. If you sister the king stud there you might run some long structural screws (like Headloks) just to ensure the new wood will support the header.

The thing that concerns me most if you want to leave the damaged wood in place is the sill plate. If that is rotted through it isn't doing its job (transferring the load of the wall, ensuring shear transfer through the sheathing, etc.) However you could probably just cut out that rotten piece and slip a new one in. This is unlikely to damage the interior since it is probably behind trimwork. You could also leave the rotted section if its small enough, and slip in 2 or more 2x4s on the horizontal over the top of it. This might spread out the load adequately.