On the Leica, definitely plan on replacing the mirror - if you've got it apart there's no reason not to. The shutter and speed mechanisms are not bad at all to service, provided you don't need new curtains. Clean everything as thoroughly as possible and lube sparingly where necessary. Needle oilers are your friends. Don't adjust curtain tension at all until everything else is done - usually you can get away without touching it once everything is clean and lubed. Slow speed adjustment can be fiddly.
On the lens, I hope for your sake that that Summitar has the round aperture and not the hexagonal one shared with the Summar. If it's the latter, start saving for the therapy sessions and medication (or booze) now. The glass is comparatively soft, so be gentle. I can't recall if those are prone to flaking black paint on the optical groups, but check it and repair as needed.
