D&M Caboose #205 is an ex-PRR N5 model. Does anyone know the approximate date the 205 was sold/conveyed to the northern Michigan railroad? Any insight would be appreciated.

D&M 205 is currently on display at the Standish, Michigan train depot.

Here's a photo from 1979

https://www.flickr.com/photos/wrongmain/36134646510

And a Railroadfan Forum on cabooses, where numbers 205-208 are mentioned to have been purchased from the PRR

https://railroadfan.com/phpbb/viewtopic.php?t=17919

I guess that they would've been purchased during the end of the PRR's independent operations, but no exact year

Is there a document regarding the D&M's all time roster? That would be of great assistance

Without documentation, this can't be proven. However I would find it reasonable to think that when the D&M purchased a portion of railroad from Penn Central before the projected operation of Conrail, that some surplus Ex-PRR cabin cars were also conveyed to the railroad too.