Odd sized exterior door options
Some suggestions:


Regardless of the door type, a good coat of primer and exterior paint will be the better of the finishing options. 
a fiberglass or steel surface should outlast a wood one (they should both be 'trimmable' with the right saw blade)
Check to see if your door slab could benefit or be harmed with the inclusion of a storm doors. Some doors benefit (water protection) other's can be damaged by it (over heating in the sun).
consider ripping out the entire door and door jamb and installing a whole new all-in-one pre-hung exterior door. This will allow you to re-square up the opening and also likely give you the most water-tight seal.