I would highly recommend that you first of all purchase a custom made replacement window to fit the existing opening—this does not have a nailing flange and can be installed from the inside and with screws, not with nails. (No nails please.) You won’t disturb the stucco and you won’t have a nailing flange to worry about.
The beauty of a replacement window is that it will leave the existing interior trim intact and the existing buck frame of the old window in place and intact! This reduces your time and concern about the home immediately.
You are going to have to measure beforehand to get a proper fit, so that means you are going to be measuring to the existing buck frame so you can get that replacement window built to within about an eighth of inch of the width and an eighth of inch of the height of the smallest measurement, top middle and bottom.
As for removing the sashes before the installation, you can either cut the interior stops or the exterior stops, depending on your preference. However, for an older home, I would suggest that you remove the sashes from the outside, to avoid any interior damage to the paint for example.
Once you cut the exterior stops, remove the storm window, if you have one. Put the top sash halfway down and the bottom sash halfway up, and if they have the old aluminum tracks, just fold them around the sashes and then just yank the sashes out from the outside. Everything inside has been left intact.
Now, you are ready to install your new replacement window. Since there are a series of important steps, I do have the window installation steps outlined with photos on my website. It’ll be much easier than trying to visualize my set of written instructions!