I didnt see my tax bill showing a line item.

COSTS:  I would look at any-and-all things needed to run an STR including items that are often done by owners--for example, property management.  

But I would be missing items necessary for taxes, etc.

Great thanks @Michael Smythe, in your experience can you itemize the tenants that have the Resident Benefit Package? 

Hold Harmless & Indemnification clausesAs well as other items.

Highly agree with Nathan - Buildium is a Nightmare to use for STR, it is not appropriate at all and prone to a lot of errors due to the sheer number of manual items that need to be adjusted in there. 

We asked them to address 5 items that we saw as the most expensive and/or critical.

Unfortunately as good as TurboTax and other online softwares are, they don't always pick up the most beneficial items with rentals.

  $50/sqft for a full interior and functional update, $100/sqft for a full gut down to studs, $150/sqft for newcon .If these numbers make sense I would want to visit the property for a true estimate, where it would be line item work scope development. 

If you are interested, you can request to meet with them and/or submit an LOI detailing your offer on price, financing options, terms, and other items related to the purchase.If accepted, request Tax Returns for this business, and have an Accountant(reach out to me if you are looking for help with the Analytics of a business purchase) compare their financials to the Tax Return.