I like the "combat sport" tested martial arts because:
1) Training them to get better than other practitioners is one of the primary goals, and it's hard for people to get and retain students if they are not measurably good (as in, students winning competitions) teachers
2) "Street" or "self defense" are environments or goals, not styles - any "street style" or "SD style" is prone to collapsing under the weight of its intentions, and any honest teacher will generally give credit to actual fighting systems and focus on the fundamentals THEN cater the delivery to the appropriate environment or goal of the student
3) By being eclectic you get to focus on different 'ranges,' typically expressed as striking, clinch, and ground. Farther than striking is running away, or shooting distance. An intimate understanding of the core arts taught in MMA classes will make you proficient at any range in a fight. MMA is not a system or style but rather a competitive filtering system that necessitates proficiency at all ranges (except shooting, obviously).
4) Working at various levels of resistance UP TO fully resisting with training partners is crucial, fundamental, vital, I can't emphasize it enough! If your MA gym NEVER does this, it's not as good as one that does. If they do it frequently and in smart ways (i.e. dosing the resistance to the student's ability to assimilate the experience), that is more important than what "art." The thing is, the ability to deal with someone trying their hardest to stop you from dealing with them is THE fundamental goal of self defense AND sportive arts. SD-oriented training often glosses over this and does not do a great job of working with real resistance.
I would say start off doing some resistance training and cardio a few times a week, take a Southnarc class if you can (this will give you a fundamental structure and understanding of the 'street' environment and 'self-defense' goal), and during the time of general personal development, sample some "free week" trial periods at various martial arts gyms but don't join until you have tried quite a few.
The thing is, there are only so many ways human bodies can tangle, disentangle, hurt one another, and defend from attempts to hurt. MMA is Darwinian in that many arts can potentially be used if the practitioner wants to use them, but ultimately only a few, fairly refined ones, are successful. By all means dabble but I think you will be successful with MMA striking/boxing for stand-up, and BJJ for the ground. People will say "But you don't want to be on the ground..." - exactly the point! If you train there, you will be better able to stand up, prevent from being injured, and defend there.
Again, Southnarc's training is invaluable to help you shape the training you receive to best further your own goals. The BJJ/boxing advice (some wrestling thrown in is good too if you can find it) is canonical - people who have had these do better in his classes than people who haven't. So, if you want the most bang for your buck, do ECQC first. If you want to do a more gradual approach work on fitness, and sample different MA gyms until you find one you like, and take ECQC as you are able.
As a side benefit, the combat "arts" that require greater fitness levels will also pay off by reducing your risk (which is more likely than being assaulted and injured) of health issues due to a sedentary lifestyle.