As I posted in another thread, I'm trying to buy a house, but we seem to keep picking lemons (first house we had inspected after our offer was accepted appears to be sinking into the ground and the foundation cracking). The current choice needs some work on the windows (some wood swelling/degrading, seals going) and radon mitigation, but the main problem our inspection revealed is that the furnace is shot. The way these things work is, when the inspector finds something like this, you either walk away or you negotiate with the seller to either replace the furnace or give you a credit equal to replacement so you can replace it yourself. Getting the credit is probably preferred so you make sure you can get something you like, not the cheapest possible thing the seller can put in there.
The current furnace is apparently an 80% efficient one. The state will give a small tax credit (I think $250) if you get a new furnace that's 90 or 95% (I'll have to look to see which) efficient. Assuming we can get the seller to give us a credit, is it worth it in long term energy savings to put out the extra cost for the super efficient furnace? I don't know how much more the extra efficient models cost or how long it takes to make the cost back. Would be looking at a forced air gas furnace (based on the ducting that's in there currently).