I recently did a restoration on a 900 a few years ago, although it is not in service yet, it soon will be. I run a 1300 at our truck garage that partially I refurb'ed. New shafts, bushings and bearings was all it really needed at the time. There are two 520's running on the property and a DF 520 that will be refurbished here soon as well.
The 700, 900, 1100, and 1300 were a much different animal than the smaller 520 and 350 in that the fire pot rotates as the unit is firing. Fines are automatically dumped into the ash pot by way of a slotted cylinder at the bottom of the pot. The unit is fed through a cross-auger and these augers can feed from either side of the burner to suit any situation.
When you get the burner removed, be sure to check the "gearbox" at the back of the fire pot. (Not the reduction box on the blower) This is a system of bevel gears, shafts, and bearings that is responsible for rotation of the fire pot. This is known to be a heavy-wear area and if the bearings have any play in them you should replace them. The 900 gearbox I rebuilt was trashed due to the previous owner running it with the bearings shot.
I love the idea of the stainless base... you should maybe give some thought to building and marketing an aftermarket stainless base for the 520's!
