Traditional the three most important things in Real Estate are
In martial arts, the three most important things are:
(or Sifu, if you're inclined that way)
I'm coming late to this question and I think the others have provided very valuable comments, but one thing I saw missing was the piece in between making the decision and hitting the mat. That is to visit as many dojo's as you can and watch as many classes as you can.
Find a teacher with whom you're comfortable. Someone who can provide training that matches your needs. Based on your question, you're not looking for "street self-defense", and you're not looking for deep mystical alignment with the universe. Toss out any teacher that is intent on delivering those more than they are in meeting your needs. (that is the most important thing I have to say; the rest is commentary)
Watch a class. Ask yourself if you're comfortable with the amount of courtesy displayed. (some dojo's bow ever 10s, others don't do so at all). Do you see people training safely? (Yes, there are limitations in your ability to judge that until you know more, but if on your first visit it doesn't feel safe, then it is probably not going to help you achieve peace of mind).
What tone does the teacher set? Does the teacher instruct, or does he hand it off to senior students? Do the senior students pass the "comfort" test? Is the instruction more or less mystical than you want?
How do the students relate to one another? Are these people going to help you achieve peace of mind? Do they seem egotistical, arrogant, obnoxious?
Is the place clean? (tough to achieve peace of mind when you're face down in an old spill of some unidentified bodily fluid with a 350 pound stinky guy stretching your arm out of its socket.)
Is there a mix of beginners and seniors? Are you comfortable with that mix? (at the moment my dojo is very skewed towards black belts; we welcome beginners, but our retention rate is high). If you see a room full of green belts with only one black belt, that will give you one type of experience. If you see a room full of black belts with no green belts, that's another experience. Either can be OK - what is important is whether you think that will help you achieve peace of mind.
Do the students seem to like one another? At our dojo we hang out together and have lunch afterwards. Sometimes we'll show up for an extra session just because someone needs the stress relief of being thrown at the ground repeatedly. (different stroke for different folks). You don't have to be best buddies with the dojo mates, but it does go a long way towards comfort.
What's the gender balance? A room with no women, or no young people, or no old people or a room where everyone has the same (hard) body type may point to an excessively aggressive training style that may interfere with peace of mind. It would matter to me, but it might not matter to you.
Do you travel? You may wish to join a dojo that is affiliated with a larger organization so that you can visit dojo's wherever you are.
Do you want weapons work and solo work? I can't get to the dojo as often as I'd like, so I want to study arts that I can do when I'm alone (weapons work is great for that - you get a lot of feedback from the weapon).
For me, peace of mind comes from working hard till my body is exhausted, feeling safe - that my partners will watch out for my safety, and that there is more learning in front of me than there is behind me.
I'd look at all those things before I'd look at internal/external or Japanese/Chinese/Korean/whatever.