I've been experimenting this last week with exactly this.
I tried a couple of ribbon mics (Beyer m500 and MXL r40) and did not get a sound I liked, I might try again with more time.
I tried an Oktava MK012 cardioid capsule and got nice results. Then I tried a Line Audio CM3 - my favorite so far. Both are small diaphragm condensers.
The large diaphragm condensors I have tried haven't worked well yet for me (Rode NT2, Oktava MK319). I am awaiting a ISK Pro Audio Starlight to see if that does the job better.
I am liking the SDC because in general, these are the mics that have the LEAST personality. Personality tends to drive prices up
I would second the previous poster and recommend the Shure SM-81. It's a fantastic acoustic mic and usable on all kinds of sources.
Remember that, ideally, once you train the Tonedexter you'll not need the mic again for awhile (if ever). So it would be smart to purchase something you know you'll use post-Tonedexter.