A planer alone isn't going to allow you to flatten and straighten stock and that will be especially important when milling lumber for doors. You need a jointer, too. Building 15 doors is quite a task but beyond the logistics of doing the proper joinery, making jambs and installing hardware, is the issue of finding enough high quality lumber. Doors have to be reasonably flat and straight and stay that way. Most modern doors are not solid wood, although, they appear to be. They are made from stave core lumber and veneered for the reasons I've stated.
It's difficult for us to know what level of standards you want to aspire to, what your woodworking skills are and what you are looking for in terms of design, function and appearance. There is far more to what you are proposing than buying some tools and flipping a switch. I've built hundreds of homes and many times the owners wanted to do some of the work themselves. All but a rare few have been overwhelmed with basic tasks like painting. Constructing some of the millwork would have been out of the question.
Not knowing exactly what you want, how much time and money you can invest, you can look at something like pre-finished hardwood flooring. You can buy many different species of strip flooring for $3 per square foot or less. If you buy hardwood lumber, it's random length and width and in the rough. It's hard to find it for $3 sq.ft. and that's just the beginning. You still have to go through the milling process, cut tongues and grooves both on the edges and ends, relieve the backs, install, sand and finish, all of which can take weeks, maybe months, if you can work on it full time and have the necessary equipment, space and knowledge.
I'm not trying to be discouraging, just pointing up the realities of what you propose. You may have unlimited time, resources, money and don't have to meet loan deadlines, building permit limits or have to live in a construction site. If you are looking to save time and money, you might want to crunch the numbers and look at things from a practical view point.