I highly recommend building your portfolio a bit or at least giving yourself some time with the property you have to experience the economic fluctuations coming and the property management aspect of holding.

Boston is a coastal city that will likely fluctuate quite a bit.

Marking fluctuations don’t impact yoU as much if you are buying and selling at the same time. 

Reserves fluctuate.

You asked about rate fluctuation from the index..

This is a recent phenomenon that is not tested over time, recessions, wars, interest rate and employment fluctuations, locusts, floods, you get the idea.

It also carries increased risk due to greater fluctuations in market values, rental rates, and rental demand.They use leverage on cash flow plays to juice the returns and because many of those cash flow plays carry very little risk at modest to high leverage levels.To each their own though...lots of ways to make money in REI.

I currently have one property that has fluctuated from a primary residence to a rental over the last 10 years (unfortunately purchased near the peak).

I think it should be more like $500+++ since the rental rates, vacancies, and repairs will fluctuate (especially in the current environment). 

everything else is about the same in costs with some fluctuations in gas prices etc..