Considering buying a house for my son to live in with a couple of additional guys.

Make plans for supplying anything else you'll need for day-to-day living. 

If you can, go several times, so you can watch it happen from the foundation all the way up.If you live close to a store like Home Depot or Lowe's, sometimes they have free classes on "how to install a ceiling fan" or "how to paint". 

Soon though I didn’t feel alone anymore, it was the beginning of 2020 and by March I was joined by much of the world in my “what’s next” quandary.I was lucky though, I lived in suburban Colorado next to lots of park land and my wife had a secure job.

You would be much better off finding a job that can at least cover your living costs while you build your portfolio.

I used to live in the property and are in very good terms with the previous neighbors, and don't want to risk anything given the shady past/current behavior from the tenants. 

That's because a lot of things that "work" when tested can't handle the load of people living there and using them consistently and so they fail quickly.

We updated the unit cosmetically through paint and furnishings, as well as consolidated the kitchen and added more living room space.

One broker told me I need a residency visa (but couldn’t actually tell me which visa to get), another said I couldn’t get a mortgage at all (since I don’t live in the U.S.).

If this is helpful how long would we need to live there to reduce capital gains taxes before selling the property?