I've definitely considered using my primary home's equity to put towards a duplex - hoping that my wife and I can save enough capital to fund a 25% down payment on a rental, and possibly do a BRRR method with my ability to flip the interior of the home between cabinetry and the renovation skills I've learned over the years.

Go ahead and remove this contingency, but add "Subject to interior Inspection." 

. $669.9k, 1548 sq.ft, $433/sq.ft. decent interior, but nothing mind-blowing. redfin lists this one as being "hot"https://www.redfin.com/CO/Denv...

Did a an interior/exterior renovation with updated fixtures, lighting, appliances and current trend paint palette What was the outcome?

Rentometer, then Zillow to compare interiors and square footage

For that kind of interior Devore Design will likely be your best bet. 

Interiors such as floors, appliances, etc are cheap/old ones, so I really don’t care if those needs to be replaced.My questions are1.

Newer replacement windows, new exterior doors (originally the building used hollow core interior doors with a screen door as the exterior).I approached the owner of next door sometime in 2019 to see if they would be interested in selling. 

I am looking or an experienced general contractor to get the interior rehabbed so that I can rent out 3 of the bedrooms.

Complete interior rehab.