My first mirrorless camera was a Panasonic, it was a great camera, although now after having used Olympus for a long time I see lots of things I DON'T like with it - most of these are probably solved with modern versions of Panasonic cameras. I paid about 5000 SEK for it, say roughly 500 dollars.
To me it was a good beginner camera because it's quite simple in its interface (and looks quite much like Canon, that I used before) and the lack of auto ISO forced me to play around with the exposure triangle a lot more than I probably would have otherwise.
If for some reason you would want to spend more, there are rumours about a new Olympus Pen-F coming. They are very nice cameras. I almost bought one when I upgraded from the Panasonic but my husband talked me into upgrading "bigger" to what was then the newest OM-D.
I love my Olympus but I really like the size and weight of that old Panasonic! My Olympus is slightly chubbier but it's a great camera. For something cheap I'd probably choose Panasonic. I'm only familiar with micro four thirds brands myself. Many praise Sony mirrorless cameras but I know nothing about them.