One of the properties has about 50K in equity built up with a low interest rate on a 30yr loan. 

You know there has to be deferred maintenance.

I know that the city has been consistently growing without missing a beat in past 30 years, vacancy rate is 3%, rent price was increasing in past 20 years, unemployment is consistently low at 3-5% at past 20 years and so on. 

It’s under $500 so not an extreme expense but I argue that it falls under the NNN maintenance purview, responsibility of the tenant.

Price is sometimes not the most important factor for home sellers.In regards to your valuation, I would verify your expenses since I would imagine most potential buyers will question why the expenses are so low (even though you mentioned you have rented the units over market rent). 

Not too low either, you don't want to go with someone who is just focused on price as this is a quality of a not-so-great accountant.

Most people who are looking to rent only 1 or 2 bedrooms prefer no maintenance places like apartments.In general, a 6 plex should be less maintenance and upkeep than 6 separate houses.  

I have wondered though if there are government grants/tax credits for providing internet to low income tenants.

Quote from @Bonnie Low: Many people do a hybrid MTR/STR approach.

The highest interest rate I've seen the past month on seller financing is 9% and I've seen as low as 5.5%.