Start with the age of the unit you have now. The gas furnace planning page turns the install year into a planning window, and that window tells you whether this is a decision you are making now or one you are making early. Either way, the house has to be ready before the new equipment arrives.
Walk the venting path
Follow the flue from the furnace to the outside. Look for gaps at joints, sagging runs, and any section that passes through unconditioned space without insulation. Ask the installer whether the existing flue size still matches the new unit. Higher efficiency models often vent through a different path, and the proposal should say which path it uses and why.
Check the gas supply
Ask who confirms that the gas line can carry the load of the new appliance. A proposal that says only that the gas line is adequate is not enough. Ask for the sizing check to be written down, and ask who will test the pressure at the appliance once it is set.
Look at the return air
Open the return grille and see where the air comes from. A furnace starved for return air runs hot and wears early. Ask whether the return duct will be resized or left as is, and whether the filter location will change. Note any closet door that is the only route for return air.
Confirm the drain and the controls
Condensing furnaces produce water that must reach a drain. Find where that drain will go and whether a pump is needed. Check the thermostat wiring too, because a new unit may need a wire that the old one never used.
Compare the proposals on the same terms
Put each written proposal next to the others and mark what is included, what is removed, and who is responsible for each check above. Where two proposals differ, ask the installer to explain the difference in writing. A clear written scope is the most useful thing you can have before the work starts.