Before I respond to your questions, here are a couple examples of the differences between the CM-32L+ modules and a regular first-gen MT-32:
Dune II - Credits (CM-32L)
Dune II Credits (MT-32)
I'm not sure of the pricing. I think the SC-55ST was just a more compact version of the SC-55 without a lot of the front panel controls (sort of like the CM-series modules). I
do know that the MKII has more voice polyphony than the regular, though. The XT looks exactly the same as the ST so I'm not sure what the exact differences are without seeing the specs and features of each module. I've never heard of the XT before now. They might be exactly the same but one may just be a later model.
There are few games that actually take advantage of the extra polyphony on the MKII and if you were to play these games on a regular SC-55 then you would hear some missing instrument voices every now and again. All Sierra games don't require it, though.
But again, as far as pricing goes it varies nowadays, since the only way to get a hold of them is on eBay. Right now I see both an ST and an XT each for $1 bids right now.
So basically, it'd be nice to get a MKII so you wouldn't have to worry about any games taking advantage of the extra polyphony, but if you can't find/afford one you won't be sorry with a regular SC-55. Also, I believe the MKII is the
only Roland GS module that has the extra polyphony voices (28 voices as opposed to 24 on the regular models). So any other Roland GS module (CM-300, CM-500, SC-55, SC-55ST/XT) only have the 24 voice polyphony.
Alternatively, there are other SC modules out there. There are the internal versions of the SC-55 called the SCC-1. I believe there were 3 revisions named the SCC-1, SCC-1A, and the SCC-1B. One (or two?) of them contain the extra polyphony of the SC-55MKII but I don't know which one(s) they/it are/is. The SC-88 module is fully backwards compatible with the SC-55 and in addition to extra instruments has 64-voice polyphony. The SC-880 also has 64-voice polyphony and is basically the simple version of the SC-88 (like the SC-55ST/XT/CM-300/CM-500 without front controls) and was cheaper. Also there is the SC-88pro which has 64-voice polyphony, even more instruments, the capability of adding insertion effects on instruments, and is also full backwards compatible with the SC-55. Then there's the big ones: the SC-8820 and SC-8850. These have 128-voice polyphony and have absolutely stunning sounds. I've been after one of these for years but have never had the money. You could say that the 8820 is a smaller-scale toned down version of the 8850 but there are some differences in the sound. I believe both are backwards compatible with the SC-55 as well, however. I can't wait to finally own the 8850.
But anyway, food for thought again. For your basic DOS gaming needs an SC-55MKII is the most you'll ever need. And you'll be very happy with a standard SC-55 model if you can't get one.