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run a flat bar?
MTB hubs?
160mm rear rotor and 180mm front?
Put a rear rack on?
Year of frame unknown but is bright green with steel forks, at 6ft but shrinking 58cm seems about right?
Yes, yes, yes, not sure and yes
6foot and ran a 56cm with a 90mm stem
Flat bar - probably.
MTB hubs - yes
160/180 discs - might be optimistic. 160, yes, as far as I know. 180, not sure the fork was designed for them that big. I'm running 140s front and rear on my (carbon forked) X and it's more than enough for me.
Rack - sort of depends on the generation of the frame. The last iteration (v3) had rack eyelets. Not sure earlier ones did.
Size - I'm 5'9" and comfortably ride a 54. You'll probably be right on a 58.
My 1st gen will only fit 140 rotor at the rear