Patching and Plastering - Sheetrock section repair

I bought a rental home the owner let go cheap. He was getting out of the rental business. We got the house with the knowledge there was going to be a bunch of fix it involved.
I am retired, for the most part, and thus diy fix it is somewhat new to me.
My question involves the wall in the bedroom, under the window.
I noticed it was black and brown and musty smelling. From what I can gather, there was an AC unit in that window.
As soon as I started to cut out that section virtually the entire section underneath the window and to the floor (maybe a foot distance) came apart in chunks.
Can a new section be put in that area without taking out the window sill, or the window, or base trim or whatever that is called, around the floor at the wall?
I saw a posting about putting in some braces to have something to put the sheetrock against.
Will that inner space need any special treatment before installing the new wall section?
I appreciate any suggestions pertaining to this, and thanks in advance.
Sarge

Make sure the wood and any insulation in the wall are ok first. Easiest thing to do is cut back to a stud to put in the new rock without having to add support. If you can put the rock all the way to the top of the opening without anything being in the way, you can probably get by without taking the baseboard off, just make sure you get all the rock out behind it. It's probably easier, though, to remove the baseboard. You'll need to mud and tape (paper tape, not mesh) the joints, then prime and paint.

Get all the old material out first. It sounds like long term leakage has caused mold and mildew to form. Both are most unhealthy.