I've got an old glass sliding door hanging around from a renovation project. Would it be at all possible to take one of the door panels, turn it sideways, and install it into the exterior wall to make it into a large picture window? I have a lot of experience installing windows and doors, but no experience custom making my own windows. Is there anything to prohibit me from doing this?
I would check to see if the OLD door had any drain holes at the bottom.
How sound is the frame around the glass [ wood, Vinyl] I would not use an Alum. Frame.
What type of glass ???? and is it insulated and clear?
Depends on the design/condition of the door. Usually sliding doors do not have equally sized stile and rails, so it might look weird. From an installation perspective, it is basically a "dead-lite" install, so no big deal there.