Can't answer your question directly but if you do end up getting it I have an SC-55mk2 which you are welcome to borrow for comparative purposes 😀

Yes, the SC-55ST is a stripped down version of the SC-55MKII. It uses the same mainboard, but doesn't have a display or control panel. Other than that, they're the same unit.
I'm not aware of any game that supports the MKII's extra features, so you won't really notice much of a difference compared to a regular SC-55.

OK, got the sucker home and I don't have a MIDI cable for it!

I do, however, have a y-adaptor that goes from gameport to two female 5-pin DIN plugs. I was formerly using it to connect my SB16 to my MT-32. Do you reckon the cable from that to the synth would just be a "straight through" cable with the DIN male plugs on either end?

Edit: Looks like it is, I should be able to make one pretty easily. Sweet.

http://www.beriomidi.info/eng/cable_e.htm

Yes that will work just fine!

Apart from that you can purchase game port to MIDI cables on eBay.

The y-splitter came with the wife's electric piano, I think it contains the electronics to isolate the MIDI device from the game port. I got the plugs and some 5-way foil shielded data cable, cost about $4 all up and took 5 mins to make.

I can also now confirm that the SC-55ST sounds identical to the recordings of the SC-55 on swaaye's site. It's a very nice sounding synth! 😀