over the past year i have installed 12 replacement windows, 2 exterior doors, and 5 prehung interior doors. I have about average handyman skills. The windows were super easy and the doors were a little more difficult. The only thing you have to worry about is getting the doors plumb and level. Beyond that, it is really easy to do yourself.
You could probably do all of your replacement windows in one day (i did 10 in a day). If you are doing interior doors I would be willing to bet you could probably get at least 5 done in a day.
To measure for replacement windows, just measure from jamb to jamb and sill to sill. They recommend measuring in 3 places and taking the shortest measurement. It doesn't need to be horribly accurate because you'll be filling the gaps with shims and then caulk.