Re: Adding a door, mortise the hinge side jamb?
If you are fairly handy, you could mortise a door yourself, if you are not, then building a door frame and mortising it will be a monumental task. What I would recommend is to take the door to a door company and have them frame/set the hinges it for you, then all you have to do is set the door into the opening. Interior doors are easy, anyone can do it, exterior door installations require some skill, so again, if you're not an experienced DIY'r, hire some help.
An added note on an exterior door, replacing one slab for another is an easy proposition, building a frame, threshold, weather stripping, and butting the hinges from scratch is not something to be taken lightly. Get help.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!