I'd say a couple of million is the minimum but you may already have that handled via your umbrella policy. 

Welcome to the Bigger Pockets universe @Isaac Feil !

@Anthony Vargas, welcome to Bigger Pockets.

This is my first post on bigger pockets though I've browsed the blog a lot and have read one of Brandon's books and will likely read more.

When you know the facts (like if this is a good investment or not, if it would cashflow, what is the best strategy to use in such a case) you will be in a better position to decide if you even want these properties.I would also recommend you, or your family members, read @Mindy Jensen's book about selling a home, available on Bigger Pockets. https://www.amazon.com/Mindy-Jensen/e/B078NHMXVTGood luck!

We're already looking to go bigger next time.  

Is it prudent to add a walk in shower in the bigger bathroom and remove the tub?

Never calculate hoping for appreciation - that's the cherry on top.The thing is, when we feel the squeeze in the RE strategy we practice, it is definitely time to step back and look at the bigger picture.

It also helps to build up a variety of team players for your future endeavors by checking out the company page here on Bigger Pockets to hook up with a few professionals in the area you need team support in.

Up to 5 or 6 houses you can get a very affordable "umbrella policy" which will cover everything in your life up to several million.