Because it's standard in most contracts to have an inspection period I make my offers based on the photo and the asking price from the ARV.

You will likely see a fairly standard band of costs for comparable services.  

Scenario 1: 5/1 ARMRate: 5.675%Monthly Payment: $5100Prepayment Penalty: 3/2/1Closing Cost: Broker Fees: $11,016 (1.25%)Lender Fees: ~$2,000Loan Fees: $600Plus standard title and other feesScenario 2: 30 year fixedRate: 7.375%Monthly Payment: $6086Prepayment Penalty: 5/4/3/2/1Closing Cost: Broker Fees: $11,016 (1.25%)Lender Fees: ~$2,000Loan Fees: $0Plus standard title and other fees5/1 ARM looks attractive but not sure if that's what other people opt for that for a STR property? 

Not only is the demands of each buyer going to vary by market and submarket, but also asset by asset.My favorite example of this was an old season of Flip or Flip, where they bought a house in LA county, versus their standard stomping grounds of Orange County.  

Instead of needing to meet DTI, income, etc qualification standards, DSCR loans qualify based on the asset itself.

Notice I didn’t suggest that max retail value is the standard. 

Generally most people you speak with are going to provide you a quote with their "standard" commission built in and anything less would be them taking a reduced commission to win your business. 

Their amenities are standard.

I was thinking about an strategy of doing Cash Out Refinicing putting that money in the stock market (distributed in ETF and some "safer" stocks) to get around a 6% anual return (estimating that low because is the "standard" average, however my current stock portafolio has a cummulative return of 66% in the last 4 years).

I get a standard bill from the city every month which makes it easy to allocate.