They also have a much more modern look to the software and they have a good Mobile app geared specifically for tenants.

I acquired the four-plex with varied levels of updates, but would need/want to paint cabinets, paint doors, trim, walls; if carpeted, replace carpet with LVP; replace faucets with modern faucets, etc.

We have 3 bed /2 bath condo that is modern with nice appliances.Besides putting it on Airbnb and letting bookings roll in, how can I reach out to promote this property? 

I spend about 20-30 min a week for each home, the rest is completely automated to give you a bench mark. 

There are some solid advantages here: new construction typically means lower maintenance costs upfront, modern amenities that attract quality tenants, and potentially higher rental income.

The quality of modern lumber is simply far inferior to the quality of lumber they were using to build homes in the early and mid-20th century.

Complicated rooflines, modern details that require a bunch of steel framing, excessively large great rooms, spa-like bathrooms, etc would push you into the $1,000-2,000 range.  

If you can get a property with a more modern floorplan at a similar price I would go for that.

Any pointers as to where I can get some benchmark answers for these questions?

An existing kitchen layout might work well for adding a dishwsher or microwave hood for example allowing it to be made functionally modern without a major rehab.