Friends who own an M6 use twice as much gas as me. I get about 15mpg with mostly enthusiastically driven city driving and my friends are getting about 8mpg in their M6.
The OP is thinking of manual so there should be no problems transmission-wise. I would steer clear of the SMG transmission. I have SMG in my track car but I would not want it in my daily driver.
I believe the M6 is reasonably reliable but I would stay clear of high mileage cars. Motor work is very expensive.
Straight lines are for fast cars...turns are for fast drivers
'04 M3 - track
'06 650i Cab - daily driver
'02 Highlander - kid hauler
'01 330Ci - winter beater