Of course we know 😀 It's all there already. Lot's of forum members think, flash is close to a sacrileg on a rangefinder body, so you collect much more replies when asking for opinions on a high speed lens. Personally I think a flash has it's uses, especially if used properly.
For the m8, I'd skip any idea with TTL and the Metz 54. Doesn't work reliably here. More importantly, pre-flash is evil in real live shooting and unneccessary in still photography. Really.
My recommendation for M-Flashes (admittedly a vee bit Nikon-leaning)
SB-22(s) (for it's size), three f-stops, if you're willing to take the "motor" stop, AA battery form factor. US$ 20-30 used
SB-27 (for the best compromise), five or maybe six f-stops, beam splitting (really works to some extend), AA battery form factor. US$ 50 used
SB-26, SB-800, MZ54i (for the best features), all possible f-stops, slave mode - forget the Metz in dedicated TTL mode. US$ 100 used and up , much more for models still in production
I've got a SB-22, 24, 27 and a MZ54i.
Rgds
Ivo