I asked a member of the RCMP why the Force chose five-inch M-10's, which they wore from 1954-the 1990's when a DA-only 9mm replaced them.
He shrugged and said that someone up top probably thought they offered the best compromise between four and six-inch barrels. A good answer. And I think the holsters looked best in their uniforms, particularly the red dress coats.
Holsters for five-inch M-10's are uncommon in the US, and you'll probably need to get one made. I suggest that you contact El Paso Saddlery and see if they have what you need. They'll make custom lengths.
If a seller wants a premium for a five-inch barrel, tell him you won't pay it, unless his particular gun is about new. And then, don't pay much more. Remind him that you'll have to have a holster made and that most buyers overwhelmingly prefer a four-inch barrel. His gun will be harder to sell, and he'll have to find an enthusiast willing to buy a five-incher.
The only rub with a five-inch gun is that they weren't made in stainless. But if you avoid oceans and rain forests and use a good lined holster to prevent finish wear, that's not as great a burden as it may seem.