If you have no idea yet, I'd say read everything you can get your hands on and as soon as something you read seems to resonate with you or catch you more than others, then use that as guidance towards tailoring what you continue to read.

After that it gets very tough to sustain without burning out fast.Really need about 70 doors or more for scale to get passive.You can use a manager at the lower unit levels but generally do not have enough gross income to pay for a full time repair person onsite.

You are following your plan and getting results that appear to be building up to a sustainable business operation.  

If you aren't there yet, I would find some means to save this money up so you can sustain your business in the short term.500 letters x .75 (cost per letter) x 6 months = 2,250$ in mailing expenses.

Your post really resonated with me, as I am also a physician, but somewhat more early in my career.

Then for you I'd suggest just a quick site search for meetup + the location you are looking for, or be aggressive in updating a meetup group, a lot of meetups get started here and attract members before becoming self-sustaining and stick to meetup.com

You might find an opportunity that you can do this once or twice, but it's not a sustainable business plan.  

The tenant cannot refuse a resonable request (i.e. during normal business hours), given this notice, and common sense applies.

Even if the numbers work out, the sustainability of those numbers after the fact is another story based on the factors I already mentioned.If it quacks like a duck...

That way regardless of what happens to the market your property will cash flow, or at least sustain itself and you won't be screwed and bleeding out like most people.