Carefully remove your inside casings to determine the rough opening when ordering the door/sidelight unit. When the door is to be delivered, have help to get the new unit set in place. When doing that size door, I have another friend with me and utilize the two delivery guys to carry the door right to the ready opening and tilt it right into place. Be sure to put a heavy bead of silicone/acrylic calk on the sill to prevent insects from getting under the sill plate. Have a level ready and plenty of shims, and be sure to center the door between the siding. This will insure that you can reuse the interior casing.
When removing the existing unit, be careful removing the caulk around the brick molding so you don't damage your siding. I use an inexpensive
Multifunction Power Tool from Harbor Freight Tools to cut the caulk away from the brick mold, then carefully clean the caulk from the siding with a sharp chisel. HFT has the tool in their March sale flyer for $19.99. I use the 1-1/2" wood blade for this task.
If you have aluminum clading on the existing door, you will need to have someone with a siding brake to shape new trim if you wish to use it. I order all of my doors with Azek solid vinyl brick molding so it never needs painting.
You can also order your door and have it installed by Pella. They will insure the job is done perfectly, and will even paint the inside and outside in your choice of color. They use Sherwin Williams paints. It's not cheap, but it's all done in one day and it's guaranteed. If you have the resources, this is the best avenue, IMHO.