I think that starting your own practice in derm is really not that hard and I have never met anyone who did it who was unhappy with their decision. However most of the people who I know well that did this, did it a while ago. I'd be a little more reluctant about starting a solo derm practice today, but not that much more reluctant. Here's a few things to keep in mind:
1. I wouldn't worry about the expense too much. You can get loans for everything. The only downside is that you'll probably need to keep living like a resident for a year or two so you can get out from under the loans fast and then you'll start to be making real money. Basically it's just delaying gratification for 1-2 years.
2. Going to a place that is underserved will make everything infinitely easier. But I think there are niches in most large, desirable cities where a derm can start a good solo practice. You just have to be a little flexible about where in that particular city you would practice.
3. If you like teaching and research, there are still opportunities in private practice (e.g. volunteer faculty or doing clinical trials in your practice). If you really love teaching and research, then just taking a full-time academic position is best.
4. Running a practice well is not really mentally a very challenging thing to do. However, it is labor-intensive. There is only so much you can delegate and everything that you do delegate you still need to monitor. Of course, the upside is that everything is done just as you want it. That's a freedom that you won't have in other practice settings. Depending on your personality type this could be worth a lot or very little.
5. By far, the most common reason why people incorporate mid-levels into their private practices is to make more money. The issue of mid-levels is multifaceted and more complicated than I want to get into right now. However, in your case you don't need to worry about that. If you're starting out on your own it will probably be at least 2 years before you'll want one. At that time, adding one shouldn't be a problem if that is what you decide to do.