I bought 15 unit and have slowly crept up to 40-50 units. 

in my mind if your just talking 1 to 4 units  those should be able to be vetted in your head or on the back of a napkin in about 30 seconds..

Being 3 units, the valuation will come from comps not from cap rate/ income based approach.

Sewer and water in my area is around 10k a unit.

You lock in your interest rate and if rates drop you can prepay at any time and refinance at a lower rate without penalty.If the deal is commercial, whether commercial multifamily (5+ units), office, industrial, self storage, hotel, you name it...the choice becomes far more difficult. 

As far as self managing, it’s fairly easy.

It seems like there are so many limitations to buying real estate with self-directed IRAs. 

On paper they were amazing deals, in reality they sucked so badly I went into self storage instead.

We Normally put steel posts nearby so no one runs into them.In the third picture one unit is just in front of the picture, then a second unit is at the far end on a post.These can be set off by deep fog, rain or sleet. 

So choose wisely and good luck I like the passive nature myself of boat and RV storage.