Amazing... Seems like the love for astronomy and synths go hand-in-hand? Thought I was the only closet (digital) synth nut!
Yamaha CS-01 (analog), PSR510, CS1x, QY70, SW60XG, SW1000XG, DJX, CS6r and EX5r
Casio FZ1, SK1 (2x)
Kawai K1m, Suzuki SX500
Akai S950 (2x)
Keytec CTS-1000g
Korg Z1
Had to let go of most of these when I started moving around too much, now a proud owner of a Korg M3 to scratch the occasional itch though
On-topic; Marc, IMHO, you can't really go wrong with anything second hand Yamaha, Roland or Korg post 2000s or even very late 90s. By then they had the sounds down pat and you'll be able to get some very expressive synths and/or pianos for peanuts. By now, gear from that era hold its value quite well too, so if it doesn't work out, you'll probably be able to get the same price you paid for it.
Things have come a long way in soft-synth land (I've coded some in my time), but I still don't like them for live noodling. Sometimes they can be laggy depending on your hardware and you'll want to make it easy for daughter to just 'turn it on' and play without hassle.
Good luck!