ms2000 is a great choice w/ the flexible onboard delay and phaser/chorus, plus 16-step sequencer for drones and tinkles.
All JV/XV synths are very capable, if not so much fun to program.
the Matrix1000 is one my absolute FAVORITES for ambient - there is no other box that cheap giving smooth creamy pads like this.
It's loooong attack EG is perfect for dramatic/mysterious swells and fading fx. For bread & butter it's too limited, but for that extra touch and warmth you won't get from any VA it's supreme at that $.
No, you are not going to 'program' it in real time, but as a "fill-out" synth for textures and beauty it will deliver the love. There are dozens if not well over a hundred presets in there that will work well, especially with outboard fx to add some extra air. Plus you can program it via Sysex - far from ideal, but you're on a budget.
matrix1000 thru outboard delay - ambient/pad sounds:
http://www.awrongturn.com/shaft9000demo ... IX1000.mp3
but imho the best synth so far for ambient has got to be the Nova series - particularly the keyboard versions - with top-level fx knobs, multimode filter+comb, 3 endlessly modulatable oscs, unbeatable arp/seq, outstanding FM and multi layering - when well-programmed it sounds a lot like a JD-800 on 'roids w/ more 'analog character' if needed.
Supernova2 direct no outboard fx in performance mode (live):
a Nova can do the same sounds as well