Looking update home heating/cooling systems due to age

Actually heat pumps can work well below 35F. It depends on humidity for one. The efficiency does drop. About 25F and below they are really iffy, plus or minus.

You CAN gain some with high efficiency LPG

A lot of your decision should depend on energy costs in your area, IE BTU/ per unit cost. The local power company should be able to give you that figure at least for electricity for straight watts per cost. A local LPG company should be able to give you the same for that. The AFUE of the furnace, whether 90+ or 80% class will make a difference. Maintenance / repair costs on 90+% furnaces CAN be high