I’m not sure which one is the most realistic.BP Rental Property Calculator Rental Calculator: https://www.calculator.net/rental-property-calculator.htmlWealth Optimizer: https://www.rpmengage.com/wealth-optimizer Zillow: https://www.zillow.com/rental-manager/resources/rental-prope...In addition to the ones listed above, I’ve also downloaded Excel spreadsheets from a couple of other sources.

Excellent credit and a 30% downpayment. 

However, I would write down in an excel sheet all of them and process that go into a rehab and do a estimation column and write-down those numbers. 

If you have it under contract  and tell them You’ll be having the  work done in the next 2 months most of the time it’s a free estimate since they’re trying to win the business.

It can be the difference between losing money and owning a winning property.

Personally a cash-out refi could help me get more quickly into all-cash BRRRs or a REI venture in Spain, however I don't know to what extent this is at the expense of rental cash flows, and what is the wiser decision long term.I've combed through posts online and looked for Excel templates to run some calcs and get a sense, but nothing really filled the void... 

Remember if you do… The STRs that win are the ones unlike any in the local market and also give the extra bit more of appeal and experience.

How could I create a win-win for the seller and myself?

A win for you, is a win for them, because your growth is theirs as well.

and it's not that anywhere is "saturated," it's that prices AND rates are high.or as @Travis Timmons said - a creative niche that you excel in will cashflow.