Although, the difficulty in finding buildings that will allow (or won't find out) about airbnb'ing can be the most challenging part!  

I have contacted them multiple times of course and a popular reason Sherman Bridge provides me is that the person in charge of my file is no longer working there.

Maybe they were less familiar with the popularity of ADUs in PDX?

It will be a challenge when your SO isn't on board (yet).I am kinda in the same boat, my SO isn't into real estate investing, but I am.

When it comes to older office, in particular multistory office buildings, the challenge is often looking beyond 'it pays a decent return' and digging in to understand what is required to day to keep vacancies low, and equally or more importantly what will be required tomorrow to attract or keep tenants?

I can tell you that my most difficult or challenging thing is contractors.  

If yes, that's where the challenges begin.

You actually can decide to reject a applicant for any reason you choose provided when challenged in court you can show the applicant you did pick was better qualified.

If you notice, some of the SFR offerings are sold out in just a few hours while others take a few days on the not so popular platforms.

An occupied property comes with additional challenges.