It's hard to say if hiring a Pro is the way to go, or if doing it yourself is the way to go. I guess it depends on your level of confidence and all that goes w/that.
I had no problem hanging a couple of hollow-core doors and bi-fold doors and pine doors in the home. But when it came to my full-oval, solid oak double-entry doors (and dual storm doors), I hired a Pro. The LAST thing I wanted to do was mess that up. It would have been too costly. Double-entry doors require a little more than 'common door knowledge'.
Now, as far as how to measure a door, it's best to know what's called the Rough Measurements. You do this by measuring (L to R) stud to stud. And by measuring (T to B) header to floor. The easiest way to possibly do this is to remove the TOP-most door trim from the wall. In a lot of cases, you can see the underlying framing. From there, you can measure your L to R, and at least know where to 'mark' for your T to B. You can measure down to the carpeting (if you have it) but the Retailer MAY want to know what's under the carpeting. IF you know what's under there, measure its thickness. And if there's MORE under that, do the same. Removing a Heat Vent/Register or something like that may reveal the 'layers' you have under there. Don't worry - The Retailer will get you the right door.
I guess I should say that the only time it would make sense to measure the Rough Out is if your Rough Out can handle a larger door (or a more 'standard' door) assuming what you have now was 'cut' fairly dramatically. Usually, there's a reason why a standard sized door was cut. So, be wary. It could be because of wiring or plumbing or ductwork or whatever. If you're NOT going to enlarge the door/doorway, just measure the EXISTING door and make life easy. The Retailer, again, will probably get you the door that's slightly bigger than what you have where you can cut it (or have it cut.) As you said, with all this, you may just want to hire a Pro.
My best to ya and hope this helps.
Jay J -Moderator
PS: If you want to know where to find a Door Pro, visit any job site and ask around. Get a couple of names. The more specialized the Pro, the more you'll pay. (You get what you pay for.) Sure, almost any General Carpenter can hang a door.)