The bottom aluminum frame (and the bottom mitered inch or so of the aluminum side frames) have blackened and deteriorated beyond cleaning or polishing. The tempered glass door and hinge are serviceable, so frugal me hates to throw out the whole door and buy new. Is it practical to repair the bottom and perhaps part of the side frames, or do I just bite the bullet and replace the door?
Not an expert, but I spend waaaay to much time on this forum reading about showers and such. No one else has responded so I'll throw in my two cents based on what I've read here.
Repairing the finish is not really an option. Due to the amount of water, constant moisture and cleaning there is not much that will last. Anything you did would be temporary at best.
You might consider replacing the damaged areas, but you would need to either contact the manufacturer for replacement parts or get very, very lucky at a habitat for humanity store or such. This would more than likely involve removing the whole unit. You mention a "tempered glass door and hinge" so I am thinking you have some sort of pivot door, so seals and making sure there are no leaks is also a concern. (I had to do repairs on our subfloor floor last year due to a pivot door that leaked.)
Biting the bullet and replacing the door is probably your best bet.