From a non-pilot point of view...
 

"It is however hard to beat for economics, get a good engineer though as they can be quite labour intensive."

From my years in ground ops with metros... very important point!! Make sure you buy the newest you can get and have someone good check it thoroughly... cheap can be nasty!

"As far as I know we are setting up new part 135 operation soon and the aircraft will be metro 3."

Just by the by... can you use a Metro 23 for 135???

>>>>>Just by the by... can you use a Metro 23 for 135???<<<<<<

Don't see why not. 19 passenger seats. Or all cargo don't see an issue.

Ok, wasnt sure how it works in Korea.. in these parts, 135 ops is limited to max 9 seats with MTOW of 12500lbs / 5700Kg and under.

Oh, in the US it's not quite the same. Under 30 pax seats can be operated 135.