Don't sign anything tonight period. Tell him you absolutely must think about it, talk to bankers, etc. Don't succumb to high pressure deals. Post back with what he says and his bid. Go out an get a couple more bids. Is this a local guy or a national company (found through an 800 #) that has a high markup and uses subcontract crews? Watch out for their deals where they offer senior citizen discounts and discounts for the first 10 houses in town to be "show" houses and by the way you may be told that those 10 are about full. If they offer discounts, then they are jacking up the price to start with.
Besides what Jay said, have the soffits and fascia done. Also wrap the windows and doors. Does he go behind the electrical meter and conduit or does he go around it?(just another place for rain to get into and rot the wood behind). Also what kind of siding do you have now?(let us know) Something like asbestos makes a difference. Is he putting insulating board behind the siding and what type and how thick? Depending upon your house condition, would he put tyvek house wrap around it first? What shape is the exterior wood in now? Covering up rotten wood won't stop the rotting underneath or any termite damage. Any unsealed places where the weather can get in, can cause damage you will never see cause of the siding.
The lowest bid I've seen around here is about $265 a square total price that includes soffits, facias, windows, doors. It is a diy project if you have the skills and know how of basic construction. Have you done any construction before? I priced materials at around $105 per square and up at home cheapo for a reference point. Actual price varies with each house. Wrapping windows and doors isn't easy and fun and you probably need a metal bending brake to do it right.
When you are done you want it to look like a new house with original siding and not something that was added on which is why it may be worth having an electrician move the meter and like Jay suggested, remove deck boards to get behind the deck.