Trust him more than me on this.
I'm just spit-balling. Polyphonic synths in his price range are tough to find. Mostly mono or para are available. Most should do double duty as midi controllers.
I'd seriously consider a keyboard and software.
The NI software I've used -
Massive is awesome, even for being as old as it is.
Massive X will eat your CPU.
Absynth is nice for atmospheric sounds, but a little weird to use and get your head around. It's a tad long in the tooth, too.
Rounds is fun, but high on CPU demand, and a little quirky to use.
FM8 is cool if you like FM synth sounds.
Monark is good for getting old Moog styles, though those tend to be monophonic.
Form, Razor, and a few others are high on the CPU demands, and tend to be very modern sounding.
Reaktor is fun, but it takes some time to sound design. The Blocks are cool, and approximate euro rack workflow. It's nice to have that available. There is also a community of Reaktor builders who put their stuff online for sharing patches. The platform is a bit weird to learn, but super powerful.