I think it doesn't matter which synth you choose for making psychill / psybient / downtempo. You can use mostly any synth for these styles.
More important is the question what you want to do with your setup:
Do you built up a studio environment or a live set?
Do you need keys and direct parameter control?
Do you want to use MIDI?
What sound you plan to create with your hardware (drums, leads, Pads, bass, Arps, FX)?
Do you need a mixer?
Do you use external or internal FX?
Do you use a sequencer?
In these days its quite easy to find cheap hardware like Volca from Korg, Reface from Yamaha or Circuit from Novation.
Just find out if you like the sound, usability and workflow of a synth and try it out.
If money matters there are a lot of possibilities to buy used hardware and test it in your studio. If you don't like it you can sell it for the same price afterwards and buy something else instead.
I hope this helps!