One thing to consider is that waste oil heaters are prohibited from residential use. Even though you're OK with doing it, and I don't see the problem either, the main thing I'd be worried about is if something goes south, even not involving the furnace, it gives your insurance company grounds to deny a claim. That to me is a bigger issue and enough to make the decision all by itself. One option, since you have a source of oil, would be to install a free standing outdoor waste oil boiler and use insulated PEX (the same way they install outdoor wood furnaces) to heat a coil in your furnace ducting. Although this could be pretty spendy, one thing you would gain is reliability and redundancy. It could easily enough be set up to where if the outdoor waste oil boiler went down, the in home hot air furnace would take over at a preset low temperature. And, if both failed, you still have a 'stock' indoor furnace that your wife or whoever could have serviced by any heating and air company that works on oil furnaces. If you had a waste oil burner in the house and it went down in your absence, a regular fuel oil tech would take one look, charge you for the call, and leave. You need to sit down and research options and cost benefit scenarios. It could be, if you're mechanically inclined, you could source a good boiler yourself and make the conversion on the existing gun, place it in a shed on your property along with a dedicated waste oil tank, and be up and running for fairly cheap. If you have to pay to have everything done, then it's going to get spendy, but with the cost of fuel oil going up and down the payback might be as little as a year or two.
