Since comps are limited, analyze similar nearby locations to estimate demand.When pitching investors, focus on a clear business case: expected income, expenses, occupancy, and ROI.

So, my first caution is it would be healthy not to expect similar results on certain real property investments/deals in the current and near future cycles.That said, there are multiple considerations and clarity needed, before I would act on any investments with your equity.

Similarly, if you put several Class D tenants in a Class A 4-plex, what do you think will happen to the property?

Then you can check AirDNA.co (or similar site). 

I am in a similar situation.

Similarly, if you put several Class D tenants in a Class A 4-plex, what do you think will happen to the property?

I am in a similar boat as you, I currently have 5 properties about an hour north in Sheboygan County. 

A friend of mine had a similar journey - they turned their old home into a rental property and soon realized they loved managing tenants and exploring ways to add value.

Are there any similar surrounding buildings?

STRs are essentially small hotels, thus they are usually taxed similarly to what hotels and motels are.