I've made several exterior doors (and
posted them on the forum). A couple of quick thoughts come to mind:
1. Unless this is a labor of love for dear friends, they'd be better off buying the door, esp since the design is relatively available.
2. While expansion can't be completely ignored, it's not really a big deal. Only the stiles contribute to expansion, you don't have the entire width of the door to consider.
3. It's very hard to make an insulated wooden door unless you're willing to deal with a MUCH thicker than normal (1.75") door.
A little more detail:
An exterior door is 1.75" thick, so you'll be working with 8/4 stock, which is rarely cheap, and often will require a fair amount of work on your part to select and carefully mill to get full thickness material for the door. So right off the bat you've got expensive wood and a fair amount of labor just to get the materials. Once you've got that, there's a lot a labor in making the door, esp if you haven't made one before. And everything is big and heavy, which slows you down. Oh, and you'll want to allow plenty of time before milling for the wood to acclimatize to your environment so it is stable once milled.
Insulated doors typically have a a thin skin of fiberglass (or maybe steel) over a foam core. In addition, there is some sort of internal frame to add stiffness. This is hard to accomplish with a wooden door. About the best I've been able to come up with is to make everything - rails, stiles, panels, from a 1" foam core with a 3/8" hardwood skin on each side. Since you don't want exposed foam edges, the sandwich has to have wood strips at all the edges. Once again, a lot of work, a lot of planning, and a result that is probably not as good as a commercial door.
Given that I've made my own doors, I think there's a case to be made for shop built doors. But that case does not include saving money, and it does not produce a door that is as well insulated as a commercial door.