I FEEL THE LIQUIFIED PETROLIUM GAS BEEING PASSES THROUGH ONE HEAT EXCHANGER [ HEATED THROUGH RADIATOR PIPE LINE ] CAN GIVE BETTER BURNER EFFICIENCY. PLEASE CLEAR MY DOUBT.
The heat exchanger may be under warranty from the manufacture, however this would need to be verify with the local supply house with your model and serial number off the furnace. I would also, question the heat exchanger crack, as some furnace have whats called a front plate. The front plate over time will crack. This is a replaceable part. And is often misdiagnosed as a cracked heat exchanger. 700 is fair for the price. I would get a carbon monoxide detector and install that. You might tell the guy to hold off and have someone else look at it. Some companies out there offer a free second opinion. You could call them and say that the first company says (your fibbing here) that you have two choices here replace the entire furnace or repair / replaced the heat exchanger. And that you would like them (the second company) to verify this and give you a second quote. Its pretty rare for the heat exchanger to crack in a residential home. Its is possible but again back to my statement above about the front plate. Just trying to give you some other things to look at and ask.
If you heat L.P.G. you raise the pressure. Is your gas regulator designed to correctly meter the fuel at the increased pressure? (There might well be some danger here.) What you propose will add the 'preheat' energy to the process, but as the L.P.G. already vaporises readily at S.T.P., you don't gain much. You might be better off preheating the combustion air than the fuel.