I've got a 1960 Ranch, 1800 sq ft, with a full basement. I've currently got an add on furnace that was built in 1980, and it heats just fine. It has a 27" deep firebox, draft blower, brick lined, and a 10"x10" door. I cut my wood 23" long give or take an inch. That seems to be the sweat spot for my stove. I split it down to about 6"~8" so that it fits through the door easy enough. If it gets down below zero, with a full load of hard wood it will burn over 10 hrs and do just fine. If I had an entire winter of that weather, I would be using about 5 cords. This winter I've used almost 4 cords, and I've burned pretty much straight through since Thanksgiving. I started burning in early October of an evening this year, and was burning all day by mid November. I run the fan on my NG furnace all winter long, but set its thermostat at 69 degrees. The wood furnace keeps the house above 72 degrees. On warmer days, I cut my rounds in half (11"~12" long). This lets me burn when the weather gets up into the 40s, but it does require that I clean my chimney more often. I brush it out and empty it once a month whether I need to or not. Takes about 15 min, and most of that time is sucking the creosote out of the bottom with my shop vac.
All in all, I'm very happy with what I have. I will need to replace my furnace in the next few years, and when I do I'll probably go with a Yukon. If you already have a forced air furnace, and especially if it's in a location where the debris from firewood is not a consern, I can't imagine a reason NOT to go with an add on furnace. The ability to utilze the existing heat ducts and cold air returns is a HUGE deal in my book.
Either way, ditch the box store idea, and get some personal recomendations on good stove shops in your area. Right now you're stuck on the idea that you want to minimize your up front costs. What you're not understanding is that for ever $100 you save on the front end, you'll spend a extra $200 every year for as long as you're el-chepo box store solution lasts, and your big-box store solution will be doing good to last half as long as a good quality product that only costs %10%~20% more. This is one of those decisions where you need to be looking well past the end of your nose. Otherwise, you're going to run smack into a brick wall.
Just my 2 bits,
Mark