I have used an AstroTrac and Polarie with my Pentax 645D. The Polarie I have not had a lot of time with--I sold my Astrotrac to purchase it. I have also use a Takahashi Sky Patrol (read expensive and heavier, but a great mount). iOpton Skytracker is a great deal. If you want high-end, then this is your guy:http://www.toast-tech.com/en/
I switched to the Polarie because it is the most compact mount. The only thing I don't like is the polar scope is not illuminated, but shining a red LED flash light down the front works. But the scope is really well design where you simply dial in the data and time to find the relative position of Polaris.
With astrophotography, you are going to have to stop down for pin-point stars. Point sources are the hardest to get nice as they will show every limitation of your lens. Usually two stops is the sweet spot.
If you want round stars without a tracker, then you are right that the focal length is the limiting factor. Then how fast you can collect light is a problem. MFD is not the best solution for this--a 35mm sensor with good ISO 6400 is much better. The sensor + technology could help you here, I think you are still only at ISO800?
Dew with also be a factor. Your cameras can get wet. I use a cover over my camera. Focusing can be tough--it is dark at night. Bugs bug you and it can get cold standing around. Take some LLCs or flat-field exposures. There are lots of good resources on the web for wide-field astrophotography (but not with MFD equipment--the usual "advice! is it cannot be done, which is not true). If you go to the medium-format section of LuLa there is an astrophotography thread.