There is a Koker furnace in the classifieds, has been there for the whole season, you may ge able to get a better than asking price on it in a month or two.
You can hook up a furnace in a primary/secondary layout, you just have to get creative with the ductwork.
A boiler is a better method as Coaledsweat mentions above. The initial cost may be higher, but worth it in the long run.
Domestic hot water uses about 25-35% of my propane year round, so when I'm running my boiler I'm saving not only house and shop heat, I'm saving on domestic hot water. It all adds up.
I dislike the propane monopoly so much that I'm thinking of figuring out a way to heat my clothes dryer and domestic hot water year-round. I will probably install solar for the domestic and install a low-tech clothesline for drying clothes.
I can just about visualize myself out hanging my socks and underwear on a clothesline.... maybe not..
Did you take a look on eBay for the water-to-air exchangers?? I'd measure your duct above your current furnace and see if you can find a 'deal ' on an exchanger.
Do you have room for a second heating appliance in your basement next to the current furnace? With a boiler you can locate it just about anywhere, even in the garage, or a remote building, and pipe the hot water to the exchanger in your ductwork.
There are some pretty good deals on ebay for circulator pumps too. Just do your homework on retail pricing, then bid accordingly.
Take care, let us know how the project is shaping up.