In my experience, a 4" doublet refractor is a perfect match for the M2. For under $800 your best bet is the AT102ED, by a pretty wide margin (IMO). Stretching your budget a bit gets you an AT102EDL, or a similar 4" f7 FPL-53 doublet from William Optics, Stellarvue, Lunt, and probably a couple others, as well as an f9 doublet from Sky-Watcher. The non-AT scopes will have to be on the used market, as I don't believe that any of them are made new anymore (with the exception of the SW, and I'm not sure about the Lunt).
The big upgrade from the AT102ED to the AT102EDL is the glass type: FK-61 vs. FCD-100. The latter will show noticeably less false color on bright targets, although the optical quality beyond CA control should be similar. The more expensive EDL also comes with a rotating focuser and a handle, which happen to be two features I personally value very highly in a refractor. The more expensive scopes I listed above all have better glass (same as/similar to the EDL), and I believe they all have a rotating focuser (not sure on the Lunt and SW).
I entertained the idea of an Edge 8 SCT on my M2 mount, but found it to be less than ideal. With a 2" diagonal and heavy EPs, balancing the scope in the dovetail shoe meant pushing it all the way up, and even then I needed to add a bit of weight to the front of the scope to really get things balanced. The whole thing seemed less stable than my 4" refractor does on the M2, especially in moderately windy conditions.
Regardless of the scope you choose, make sure that you have a steel column and the pan-handle for the M2, as well as a substantial tripod. I consider these to be mandatory accessories if you intend to use this mount to its greatest potential.
Edited by rustynpp, 27 October 2020 - 03:04 PM.