Wow, looks like this topic went unanswered. Sorry about that but this forum is seeing a lot more activity the past couple of months and I'm sure your questions will be answered more promptly from here on.
About your door, the measurements you give are not unusual at all, assuming you're measuring from the insides of the doorstops. If you measure to the jamb it should be about 36" wide by 80" tall, which is a standard opening.
Metal screen doors are designed to fit in the door jamb and allow for a little room for play, so installing one will probably require you remove the stops. Also, the height for most metal screen doors is adjustable, usually from about 79" to 80", so the threshold shouldn't be a problem.
On the other hand, if you are measuring to the inside of the jambs it's a whole nother story, in which case I'd suggest you go with a wooden screendoor and cut it down to size.
Now that this topic is active again I'm guessing you'll get more suggestions so don't run out shopping just on what I say. Give this a little time and see what advice others have to offer.