I own one. 12.5" is going to be significantly easier, given that you can buy any existing 870DM and drop a barrel on. Any 870 barrel will work on the 870DM - I used a 12.5" Dlask barrel.
The bolt is specific to the gun, and includes some small parts that I would like to have spares of - a screw and washer, plus the tab that push feeds rounds from the magazine into the chamber. The magazine well could likely be retrofitted to an existing gun, but that would require a drill press and a lot of measuring off of an existing gun, and is not likely to be worth the hassle. Otherwise, there is a modified trigger group with an enlarged bolt release, plus a feed ramp that is held in by the front screw from the magwell. The magazine tube is empty, and is effectively blocked by that same front screw.
It's a fantastic gun. I have three six round magazines for it. I'd love 10 rounders, maybe one day. I'm tempted to modify the follower to squeeze an extra round in, as it seems like it would be a very small amount of material to remove.
I recommend it over a standard 870. You can go from an unloaded to fully loaded 870 in about one second flat.
Mine was worked over by Casey at TacOrd, to include some dehorning and then cerakoate, because it is essentially an Express 870, with its standard mediocre blued finish.
Bradley