@MissEmmy in the US the Brother 1034D is very popular and reliable. The new version, the 1034DX can be bought at Walmart for $199.97. Might run you up to $300 elsewhere. You don't need a dealer. If you were buying a $5000 machine maybe.
I have a Singer that was under $200 at Wallyworld. It does all the basics and has a very narrow narrow rolled hem which is what I was after. I had taken a class with Kayla Kennington and she uses that a lot.
Anyway, as the cashier told me, there's a 90 day return period. She suggested I take it home and sew the he** out of it then bring it back if I didn't like it. It's still here. If I was doing over I'd get the Brother I mentioned. Or if my pockets weren't full of holes the newer Bernina. Drool. I took a class using it. OMG that is a sweet machine.
As for air threading. My friend was the manager of the local Bernina dealer when I was serger shopping. She told me not to get air threading. She said they were constantly having machines in for repair because the air threading went out. Threading manually is easy and really only takes minutes.
Coverstitch is another story. You would want to be sure the machine is easy to change back and forth between the two and easy to reset.