Myself if didnt have a lower window I would add one. Actually on the round to it, if I came across frame is to change our little lower window to similar one as upper.
Unless there was a structural cross piece (doubtful) exactly where you want window, especial if small sight glass, do not see how it could make door less secure or sound. Not much to most doors and many have lower window. You might need to insert a frame or blocking for the window to sandwich outer panles. Some doors (ours) are screwed together so could be disassembled, but if/when jig saw would be my 1st reach. With either a very long or very short fine tooth and lots of tape to protect surface.
Lastly though is price. Windows arent exactly cheap either. So if you otherwise dont want/need window, that might be consideration.