So I had someone non licensed do one for me in New York for one of my rental properties.
They just swapped an old Furnace for a new 80% Efficiency furnace. I got a RUUD furnace, the price of the furnace was $750, the labor was $350.
We went from about 125k BTU to 72k BTU, and increased efficiency.
Bill dropped in half for natural gas. We were much warmer.
So here is the deal, he did not replace the air return. It was too small for the new furnace. If I would put a pleated filter in the furnace, it would overheat then shut off.
Had to use a fiberglass filter and even with that the return was banging every time the furnace started up.
Also the newer furnace threw a lot of condensation into the 80 year old house's clay lined chimney. It was tearing up the bricks after about a year. Once it starts leaking CO into the attic or living area you have problems.
Options were a liner in the chimney, along with some mason work.. I was looking at that, and it was several thousand dollars, this is a 25 foot high building.
I did not end up doing that.
So, I ended up dumping the house and moving to Texas.
Right before I left the furnace needed a part, the manufacturer would not warranty because the installer was not on their approved contractor list.
I paid about $1100 installed for the new furnace, and got an 1100 dollar job. Not a good job, but working for a while.
If you want to eliminate the chimney, then go with a high efficiency furnace that vents through plastic PVC pipe.
If your chimney is still good, and is properly lined then the 80 percent standard efficiency furnace will be much better than your 30 year old model, which is probably operating at half of its rated efficiency.
Get on the HVAC Talk forum and find a contractor to do it right. They will run a calculation to size the air ductwork to the furnace, and select the proper amount of BTU's for your house.
It is a shame you are on propane, that is quite a bit of money more than natural gas. Also most natural gas companies offer cash rebates for installing energy efficient appliances, I don't think propane companies do that.
I now live in Texas, and when I checked out the gas furnace I have here installed by the builder, the return air is twice as large as what I had in New York, and the furnace is half the BTU's.
I am still a fan of the fiberglass filters, they let the air flow through the furnace better, and the furnace works less hard to suck air.
So, yeah you could probably do it by yourself. The installer did mine and it was very straightforward. But you will want those calculations done, all codes followed, permits pulled if necessary, also safety of your family is most important.
I would look on HVAC Talk and angies list and talk to some contractors.
