How do I get the rest of the group comfortable with investing in another market where we would need to source property managers, learn the area, etc?

If you are at the stage, or get to the stage, where it makes sense to split your assets across multiple LLCs there are options other than series LLCs which may meet your needs that are simpler and more cost effective (eg create a second LLC).

The house is paid off, and I'm trying to see what my best option is to do with it.I would like to do a full gut renovation on all 3 apartments, but I would like to be able to see if it's possible to convert the cellar to be a legal apartment and get a CO that states "multi-dwelling" instead of "2-family".Does anyone have any experience with this?

@Paul MeyerMost people pay cash because it's the easy option.

We offer all 3 options (4 if you include email which go to a unified inbox). 

Would setting up an S-Corp be the best option?

(source)It also states this:  The following requirements must be met by taxpayers or RPEs to qualify for this safe harbor:*Separate books and records are maintained to reflect income and expenses for each rental real estate enterprise.

Zillow is a fine place to start, but shouldn't be the only source of data.

Columbus is a great option for appreciation. 

Since that is so hard to predict, I think you should move forward if that is the best option for you right now.