Englander 28-3500 add-on

Around $1000.

HD sells them online $1250 or so delivered.

No problem. I've been looking at add on furnaces as well. There are a few in that price range. One that I've heard pretty good things about is made by Daka and sold through Menards. That one runs $799, although a little smaller than the Englander. My plan is to watch ebay and any local deals to see if I can catch something on clearance if not I guess Menards keeps the Daka's in stock year round and they occasionally go on sale.

If you're heating with propane than you could probably go a little more upscale and still get your money back pretty quick. A little bit nicer one that I've seen and like, but a little more than I want to spend is the Yukon Eagle Big Jack or Super Jack. Another nice one, and clean burning, is the PSG Caddy also sold as US Stove Co Model 1950 (w/fan) / 1900 (no fan).