A few variables here -
1. Battery - I don't know what battery you have but I would guess the factory installed battery is not the best. I would consider a marine/RV deep cycle battery (group 27) as a replacement or second backup. (I have had good service from Interstate Deep Cycle Group 27 and they are readily available)
2. Furnace - How much current does it draw and how long will it run. Current draw is probably 3-5 amps. How long it runs is dependent on the temperature and how warm you want to be.
3. Generators - There are lots of 1000-2000 watt generators available. Honda is probably the "Cadillac" but you pay for it. If this is to be an occasional use generator I would consider a lower priced model. If noise level is a concern check the sound level rating. You can probably find something in the $500-$600 range. The only problem I have is fuel consumption. I have a diesel truck so I have to carry fuel for a gas generator.
4. Alternatives - Catalytic Heater - You could consider a catalytic Heater - uses no battery power - but will use up your propane. Here's a sample from Amazon Amazon.com: Camco 57331 Olympian Wave-3 3000 BTU LP Gas Catalytic Heater: Automotive