I keep forgetting which is which of these. On the Rockheim pages (https://digitaltmuseum.no/011022857628/synthesizer) they show the sx-k100 and mention that the sx-k200 was launched at the same time. Also, they say that the sx-k100 is the one used by Knutsen & Ludvigsen, but I have to check this with my photos from the exibition.
Anyway, these are the facts:
The sx-k100 and sx-k200 seem to be the analog ones. The sx-k200 has a solo synth part that the sx-k100 lacks.
The sx-k250 is labeled "PCM keyboard" in the photo of the cartridge bay here: https://www.olx.pl/oferta/keyboard-technics-sx-k250-analog-pcm-CID751-IDmTmUI.html
The sx-k200 here http://medias.audiofanzine.com/images/normal/technics-sx-k200-301220.jpg lacks that text.
The sx-k150 is sold as a PCM keyboard on Gumtree.
Conclusion: For me, the sx-k100 or sx-k200 are the ones to get.