If it's a gas furnace, all you need to power is the low voltage circuitry and the fan to circulate the air. My furnace is very low amperage draw, and easy to start up - my 5500W generator doesn't even stutter when it starts up. It sounds like all those circuits you mentioned will run on a 3500W generator. The 7000W may just be overkill and use more fuel than needed (can be a real bummer for long outages like in hurricane or ice-storm situations). It looks like that unit is rated for 8 gallons for 11hrs on 50% load. If you don't need a unit that big, it's just going to use more fuel. My 5500W unit has a 6.5 gallon fuel tank and will run 13 hours at 50% load. It's always nice to have capacity, but it all depends on how much you want to pay to have it because you'll pay for it everytime you run it whether you need it or not.
That being said, I'd jump on that deal like a fat kid on a cupcake. Generac setups are nice and well-built. I paid $599 for my Homelite 5500W/6000W peak unit from H/D a couple of years back, so that unit for $650 is a steal!