I would suggest Nikon or Canon, because if you want to try out portraits there is a ton of gear out there build on those to photo systems. No matter what direction you take, there will be options. I’m more of a Nikon fan, but either would work.
Specifically, I would look for a used D5600 or D5500. You can probably find one for $300 or $400. If you want to spend more the D7200 is a spectacular option. That may cost you $600 to $800 used.
With the D5x00 series, you will have some limitations. The flash cannot be operated in commander mode to control an off camera flash. However there are wireless flash triggers you could use.
The Nikon AF NIKKOR 50mm f/1.8D lens costs about $100 and provides a ton of bang for it’s buck when it comes to portraits. The cool thing is if you wanted to go from APS-C to a full-frame camera like the Nikon D750 later on, that lens will still work.
I know someone is going to jump in here and start yelling about Sony’s mirrorless line, but there is just a plethora of options in the beginner Nikon or Canon camp.