Forced air wood furnace-need some help with terminology

I'll start with some background:

The house I just bought had a brick corner hearth pad in the kitchen, and an unlined masonry chimney. After lining the chimney (Ventinox SS liner and Thermix), I installed a used, older air tight forced air wood furnace I bought from a co-worker. Very simple furnace, meant to be installed in a basement or furnace room and piped into the air ducts and return air ducts. I'm going to run some duct work in my overhead crawlspace to feed the extended rooms for more even heat, but that I've got figured out...Where I come across the snag is here:

The furnace is set up with a simple spdt switch to select low fan speed or high, with a thermal switch that kicks the blower on when the firebox is hot enough, and the furnace just plugs in to an ordinary wall outlet.

   What I want to do, is use a 24v thermostat of some kind in the bedroom down the hall to control the fan speed, on at call for heat (if the thermal switch is satisfied that the firebox is hot enough), high speed fan at a 5 degree or greater difference, low at a 1-4 degree difference, and off once the thermostat is satisfied.

    I worked as a residential electrician for 4 years before I started my current job, but my exposure to hvac control terminology and devices has been limited, what would I need to make this work? I know I'll need a 24v transformer, and i'm guessing a 24v/120v fan relay, but is there anyway to operate at different fan speeds automatically? I guess I need to dig through the furnace wiring and determine how the blower speed is determined by the spdt switch, I'm guessing it closes pole 1 on the blower in low, and closes pole 1 and 2 on high, but I've not dug that far into it yet.



Thanks