AirDNA usually has very accurate data for vacation rentals, which is what I have found.

This Wikipedia page has the data you need.

Most of these items can be pulled from public data as well!

How do I go about pulling these lists that include the data I'm looking for:  Older, 5+ bed homes, ADU's, absentee/vacant owners?

Tools like Verifast or Plaid pull data straight from banks or payroll providers.

Redfin announced that in February, Milwaukee was the #1 market in the US up +20% but I would label that as a data glitch: last year Feb was quite low, which caused the spread to be bigger.

Loans to LLCs often require:Higher down paymentsHigher interest ratesCommercial underwritingMore experienced borrowersSince this is your first deal, buying personally keeps your financing options wide open — especially when it's time to refi.You're asking the right questions, and this "test drive" approach with your first BRRRR is smart.

Would I need to season the rent first or would it be possible to just use market data?

So, we obtain a full credit report and review collections, chargeoffs, age of credit accounts, active accounts, etc. to build an overall credit evaluation.FICO ScorePct of PopulationDefault Probability800 or more13.00%1.00%750-79927.00%1.00%700-74918.00%4.40%650-69915.00%8.90%600-64912.00%15.80%550-5998.00%22.50%500-5495.00%28.40%Less than 4992.00%41.00%Source: Fair Isaac CompanyPublic RecordsWe also obtain data from national databases about evictions, convictions, and sex-offender histories.

This is relatively new, so data is still being compiled, but from some of the reports I've seen, the cost of the conversion and/or adding an ADU is more than the new appraised value with multiple units on it.