I too am interested in the outdoor forced air furnace and have done hundreds of hours of research on the different brands out there. Also, have been PMing some guys about their outdoor furnaces. For me, I have narrowed it down to the Hopsco or the Shaver Thermorwind. Again, these are the two that I think will fit my situation the best.
These can tie into my existing ductwork quite easily, just need to add a cool air intake/return. The main thing is all the mess will stay outside, since I do all the cleaning.
Due to the simplicity, I am thinking of installing one of these smaller units in my great-aunts house that I am currently living in and using it for this next winter, and if I like it just undo the ductwork and take it out to my house on the farm next summer, if not I will install it in the shop to keep it warm and go another route.