OK, I might be buying a very high miles M Coupe, like next week...

100K+ right now... my question is, if I build a new motor next year for the turbo work, should I put my dash back to 0? or leave it, I dont plan on ever selling it, so I dont care what the carfax says about a reset miles indication...

I'd say leave the odo stock to indicate the wear of the car.

Document the miles when you do the swap so you can tell how many miles the engine has.