Get something with at least 61 full sized and weighted keys. None of those light weight organ type keys, they need to feel more like piano keys. Polyphonic - able to play at least 8 notes simultaneously.
That would be the bare minimum IMHO. Getting the full 88 keys will probable ensure the rest but maybe not in some cases.
Don't you kind of have to go out of your way these days to _not_ get something polyphonic?
As someone who plays a musical instrument as a hobby, getting an el cheapo toy seems a good first step to me.
You get to see whether you actually enjoy playing and it lets you become familiar with the practice schedule. When you start to feel limited by your tool, you advance to a proper one.
Otherwise you risk buying expensive gear and letting it collect dust 2 months later.
For example I bought a $200 keyboard once because I thought it would lighten my load as a gigging pianist. Nice and light, 88 keys, built in speaker. Much better than lugging around my thousands of dollars of pro gear. But when I got it, it didn't have weighted keys, ok not fun but I can deal with that, but then it didn't have velocity sensitivity... Every note was just on/off. This was a deal breaker, it is strait up not fun to play a piano like that. I mean it's in the name Piano/Forte the instrument that can play loud and soft.
I got a Cassio CPS-85 over 20 years ago. It was $900 bare minimum for me at the time and I still think the weight of the keys is a bit light. Still, that was my minimum and it still is though something like that should be cheaper today.
I had a very cheap children's keyboard with non-weighted keys before. It was totally frustrating to play on that. Weighted keys made a world of a difference!