I’d like to build a model that would be easy to plug in a property’s estimated numbers and it tell me if it will be a better flip or buy and hold investment.

@Jill Jay I believe the best person to contact to begin would be a General Contractor for an estimate if you aren't doing your own work.

First of all congrats on your first purchase   Call a couple contractors and obtain their thoughts and estimates to compare repair costs. 

My favorites are The Book on Managing Rental Properties by Brandon Turner and the The Book on Estimating Rehab Costs by J Scott.House-hacking is one of the best ways to get your first property AND gain valuable experience.

I looked up the price of his property/estimated mortgage and decided I never wanted to rent again!

Have they given you some kind of estimate?

If you plan to make any repairs or rehab, get all estimates and quotes from your contractor in writing.

The home is paid off. (3) There are back taxes $9100 estim.

Nowadays, the closing disclosures and legally required Good Faith Estimates (or whatever they are called now) actually shows the two sets of amounts.I might be missing something, but you really don't get charged "more" for closing early, or 'unfairly charged more' --- obviously its "more" since you have up to an additional month of increased carrying costs to pay for...Hope that helps. 

How do you have a good estimate for how much you expect to pay or set aside for these?