OK, I'm digging up some old memories now....
M23 is far better than the MIII with the extra 10% power. However the same W.A.T. hasles apply. Plus you'll be ZFW limited on sectors greater than 2 hrs (from memory) if not W.A.T. limited or LWt limited.
The M23 carries its water meth in the center, at the CoG whereas the MIII has it in the nose. Therefore if operating at MTOW you'll need to shove AS MUCH as possible into the nose compartment. Try to get to it's max compartment weight, therefore think small and heavy items. When flying at around FL200 you'll be thankful. Otherwise it'll be pain to trim and very "pitchy" along the lateral axis, ie porpoising.
Metros have a history of uncommanded nose wheel inputs. Know the system. It is a "serviceable unservicability" to operate the aircraft without nose wheel steering operable. Assymetric thrust and braking plus a lot of finesse!!!
Inflight fuel balancing is simple but has led to a few accidents. Ask your ground instructor....
Wheellwell fires have also caused some accidents. It'll burn the wing right off if you can't get it on the deck fast enough. Never trust the Master Caution panel if it says the fire has gone out! The fire may have burnt throught the circuitry and give false indications.
Engine bog down has cost some operators more than a few $$. DO NOT select speed levers low too early after landing! And never go into Beta mode with speeds low!! Again, you're ground instructor may spend an entire hour or 2 discussing this point. This is a disadvantage of a direct drive turbine on the Garrett.
It flies nose high so icing can be a HUGE problem. It will shed it from the intakes, prop and wing leading edges but if you get it on the belly you have no option but to descend. I learnt that lesson the hard way and was lucky to walk away.
It will not perform at all well on one engine at MTOW or even MLW. I don't care what the AFM says. Driftown will get to you FL120 at a weight between MTOW and MLW, but more likely budget for FL100 or 8000 at best. If landing, you may as well be committed as mentioned, the Approach - Go Around performance is woeful.
At very low weights be VERY careful of torque roll when recovering from a (simulated) engine failure after take off or in a go-around. Know what to do "if".
Respect the girl and she'll respect you. That ESPECIALLY goes for maintenance. Shonky maintenance and you're on borrowed time....
I liked the Metro and would like to think I'm a better pilot for it. Good luck with the endeavor, and more over, ENJOY! I hope the above helps. Bear in mind it's been more than a few years!!