My top two items that require some demo are replacing the polybutylene pipes in the unit and replacing the past-useful-life water heater on the second floor.

As a start, free online services such as Homewyze can help you understands costs for many rehab items.

What you should be looking for in your own private residence might be out of the scope for this discussion but I can comment on the other items. 

What other items should I be thinking about which would determine if it's a good deal?

Podio is very customizable so it serves as the "central hub" or "interface" and creates the relationships between items.

If that is the case, that there isn't a lot of equity in the home AND the luxury item will add no market value, then it's purely a luxury item, and probably best not to finance it at all, even if in theory you could just put it in a credit card. 

Use your bank statements and credit card receipts to itemize your true rental income and expenses.

Also, I realize the idea of buying widgets tax free so you can tell your multi widget thing after with tax included (more profits for the state), but the wording does not say anything about "multi widget item must sell sales tax".

The maximum capital gain rate of 20% most likely applies to some portion of your taxable capital gain.Other questions...Purchasing an RV to use as your primary residence will most likely increase your usual income tax bite because you will have lower itemized deductions.  

Waiting to discuss what items should be addressed before the sale moves forward to close.