Sympleman, One thing to consider with your plans.... When I was looking at adjustable stems, even those labeled as MTB Stems had warnings that they were not for "aggressive riding" to avoid stripping out the pivot point
You would need to decide if your off road use would fall under their warning.
I also recently converted two older MTB's to bike trail / casual street riding bikes.
Basically the same as Skidozer projects except I went with threaded / threadless adapters as well as adjustable stems and riser bars. I also swapped out the knobbies with High Pressure 1 1/2" street tires. I did have to dump the ultra light seat on one for something with a few more ounces of Gel padding.
I have been very happy with the suspension seat post on one of them "older USE model" I only wish the CF post on my road bike could eat up vibration and bumps like the suspension post does.
The $100 or so bucks I invested to "Comfortize" these two MB's saved me around $800+ each compared to buying a new comfort / cross bike with the same components.
Granted, something has to be said for that "New Bike Smell"