I prefer to take a different approach in my answer, than listing a few choices of objectives. I also don't know if you are already using compound microscopes. If you do, much of the following will be obvious.
At 50x, you are probably already working at the limits of what you can reasonably expect from industrial slides and micrometric screws. Assuming that the next step will be 100x, my suggestion would be to strongly consider switching to a compound microscope stand. However, using Mitutoyo M Plan Apos on a Zeiss, Leitz etc. ordinary cheap microscope stand is unfortunately impossible without serious modifications.
For use with Mitutoyo M Plan Apos you may instead look for an industrial microscope stand like the Mitutoyo FS60 and FS70 series. They are not very common on eBay and somewhat expensive, but they do pop up now and then, and require no modification unless you want to get transmitted illumination. See
http://savazzi.net/photography/mitutoyo_fs-60.htm
A measuring microscope stand may also work, as long as it is equipped (or can be modified without too much fuss) with coarse + fine focus. Working at 100x requires a good alignment and tight tolerances, however, so DIY solutions that work well at lower magnifications may not be precise enough at 100x. My preference would go to a ready-to-use stand.
You will need fine focusing on the order of (preferably) 100 microns per turn of the fine focus knob, or (at a minimum) 200 microns per turn. There are good reasons why virtually all compound microscopes use these focusing racks.
Since you already have a 50x Mitutoyo, keep in mind that using it on a microscope stand will be much easier and more enjoyable than on a micrometric rail. If you have other Mitutoyo M Plan Apos, then a Mitutoyo scope will make working with them a real pleasure. It would also make a Mitutoyo M Plan Apo 100 your next logical choice.
PS - keep in mind that the Mitutoyo M Plan Apo SL series has higher working distance and lower NA than the "ordinary" M Plan Apo series. The impact of the NA difference becomes more significant at higher magnification. Also, if you mix SL and non-SL objectives on the same scope they
might not be parfocal (I am not completely sure about this).
