Welcome to the Bunnings Workshop community. Thank you so much for joining us and sharing your wooden bay windows project.
Thank you for sharing those photos with the community. If you are planning on purchasing a new tool to do your project, I suggest getting one that is in line with your current toolset. If for example, you have a Ryobi Cordless Tool it has a battery system called ONE+ that allows you to use the battery for the entire tool range. I recommend the Ryobi One+ 18V Corner Sander, it is compact and designed to reach small tight spaces.
I recommend examining the condition of the putty on the glass windows and see if they need to be repaired. I suggest using Selleys 450g Linseed Oil Special Putty to replace loose and craked pieces on the window. Once repaired you can then begin sanding the window frame. Please note that I recommend scheduling this type of repair with the weather forecasting at least several days of sunshine.