Mark, I'm really not familiar with PianoBox but as its a general midi synth I'm not sure it will meet your original spec of having a "decent range of spacey synth sounds" and "a few organ sounds along the lines of the B3, plus I need some nice acoustic Grand Piano sounds". While it may have some of these, I'm not sure you would find better pianos than what's already on your Casio.

A route that you might want to investigate is SampleTank 2 XL. This is a software based sampler in your price range. Here's a video: https://www.youtube.com/watch?v=rHIqLVnTHvM

Win11, Intel i7 7700K 4.2Ghz, 32Gb RAM, 2x1Tb HD, 500Gb NVMe, BIAB/RB 2025, MOTU 828MK3 audio, MOTU Midi Express, Yamaha Montage 7, DX7II, TX802, Motif XS Rack, Roland Fantom XR Rack, Oberheim Matrix 1000, VoiceLive3 Extreme, Kontakt 6, SampleTank 4.3