I'm actually undergoing the exact same process right now. I'm designing in CAD and 3D-printing my parts, which makes it a bit easier to adapt unusual lenses (since I can print the bayonet needed).
I think for Micro Four Thirds, medium format might be a bit overkill. You'll have a truly enormous image circle, so you'll essentially never run into the limits of it with reasonable deflections (mine has +-12mm shift and about +-10 degrees of tilt), and using a Pentax Takumar 35mm/f3.5 I haven't experienced excessive vignetting.
However, my main reason for fussing with tilt-shift is to experiment with converging verticals and interesting landscape effects, so I wanted to go much wider. Which is going to be impossible to find with medium format (unless you're spending a fortune), and somewhat difficult even with old film SLR lenses. I eBayed a 20mm/f2.8 Cambron lens with a Konica mount for about $30 (+$30 shipping), which seemed like a great deal for what is essentially an ultrawide on FF. That still only gives a 40mm equivalent viewpoint on M4/3, so it has some limitations, but I'm used to that from my Panasonic 20mm/1.7, so I think it's worth the experiment still.
Unless you're looking at doing very technical product photography (which I'm guessing is unlikely with a DIY adapter), or unique portraiture, I'd recommend going as wide as you can affordably get. I'd put 24-28mm as the longest focal lengths to look for, with the wider the better. But that's just me!
you'll find that tilt has the most profound effect increasing from normal to tele and shift has the most profound effect in wide decreasingly to normal.
Given the size of the sensor coverage from 35mm lenses gives plenty of movement (more in fact than my large format lenses on my 4x5 camera. There is even less reason to go to medium format lenses because they are bulky and often not as sharp as 35mm lenses are for the same focal length and cost. This is because when they were designed they were not intended to magnify as much as a 35mm was.
36x24mm capture to 10x8inch print (which btw is a missmatch of aspect ratio always resulting in cropping) is about a x10 magnification while a 6x9 is only about x3 or x4 magnification
looking at what tilt can do for a normal (Sigma 30mm vs Oly OM28mm tilted).
Which is a third reason to not get into medium format, as the focal length needed for wide angles there is something like 60mm while 60mm is quite tele on the smaller format. So unless you want to have an adapter that's used for more or less 1 lens (and its not really handy in super telephoto) there isn't much on the menu when choosing from the medium format restaurant.
Personally I am happy to have one adapter that does tilt and one that does shift, for I use them both differently and I associate each with a focal length group (lumping wides into one group and tele's into another).
Lastly, my primary use of shift has been perspective correction, which I now do mainly with multi shot panos (to get enough data) and software (like PTGui).
For instance this shot is a stitched together HDRI to capture an interior and not blow out the stained glass (and thank god these people just chatted for the whole time)
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PS: long after I wrote that blog post about adapter ideas I bought a ball type tilt adapter (which I was using in the second article) and find that its bloody cumbersome to use.
Now I've owned TS-E 24 and 90mm lenses and the Canon method (like the russian makers copy of that mentioned in there) is the best because it allows you to tilt without throwing the lens center distance off from the sensor plane. Meaning you don't need to refocus just because you apply tilt ... its a bit like the Sinar method in 4x5 studio cameras (which I'm sure Canon were aware of).