We've been making our own doors (in our smallish custom shop - 3 to 4 person, 6000 sq ft) since the early 70's.
Always have used a good 5 or 7.5HP shaper to machine the frames (with a tenoning attachment or Weaver set up) and raised panels.
If you are processing your own rough lumber, then all you really need for your quantity of doors is a decent chop saw (we use a 5 HP, 14" radial arm saw set up for just cross cutting), a jointer, planer and table saw.
Wide belt sander is pretty much mandatory IMO to get a decent flat door in good time.
With a decent selection of cutters for the cope& stick (frame) and raised panels (I dont buy anything now except carbide insert tooling) you can do several door styles all using the same heads.
Realistically, one man in my shop can easily produce 50 doors in a regular 40 hr week, from rough lumber to a finished, sanded door ready for the finishing room. Easily 100/month.
Going up in volume, I think you would have to investigate specialized machinery - for frame and panel processing, as well as more automated machinery to prepare your stock (moulder, coping or mitre machine, profile sanders, etc) but you will be looking at a much larger outlay of $$.
For door frame stock, I always rough cut my material, let it sit for a day, then joint and plane before cutting to length and shaping.
Door panels are laminated, sized and thicknessed, backs pre sanded, shaped, sanded and clearance taken before assembly. The the assembled door is edge sanded and run though the widebelt before hand sanding.
Pretty straight forward process.
For mitred doors, you can set yourself up for assembly with loose tenons. Slot mortiser and machine your own tenon stock.