One mode of thinking is that the cheaper rentals suffer less from a downturn, since "they gotta live somewhere" and some renters from the higher end units will move down until the economy rebounds. 

If any move out I’ll need to make some improvements to get market rates, about $4500 per unit for the cheaper two.

It will be a lot cheaper for labor costs?

If you market they will come but it's gotten to the point where you have to cold call unless you want to spend thousands for marketing thereby requiring you to get the SFRs cheaper. 

The cheaper that interest is and the reduceed  amount of time you're paying on it e.g. not when you're not using it makes the cost of that money less. you can spin scenarios where the original loan gets rolled over and over and that is fine but you're still paying interest on that and if you're having successful real estate Investments ultimately making money then great we just made money and paid for loan costs.   

I own a bunch of them although i bought earlier and cheaper.

Building is cheaper than buying in my area.

Companies like Doran, Sherman, etc are getting gov't bonds and tax credits as well as cheaper institutional type investors for the capital stack. 

I can't tell you how many times in my life I have gone the cheaper route, on cars and houses, only to have to come back in six months or a year and having to pay again to "do it right" and replace that used part with the new one.

A Heloc is about $4k cheaper and WAY easier to obtain than a refi.