We're pretty well all still in the dark about your exisiting heat distribution system and layout of it. Also your basement & garage size & layouts. Regardless of what you do for a new furnace - three full tanks of oil in the basement with the furnace they are hooked to in a garage on another level (or maybe its not on another level?) just doesn't seem right to me to start with. One small oil leak & your house would be pretty well toast. I would get them out & replace with a single one close to the existing furnace, and put my new wood burner in the basement. Again, that's without a clear picture of your entire situation.