You should ask him if he'll take 450, then take it for 650 if he's firm. As was said, it's worth more as parts than the price so you won't lose money on the deal. I also wouldn't worry about mismatched parts on a 14 year old project car. For one, after all that time on a car that has no parts made for it anymore people have to find what they can. For another, look at people here putting Gen 2 interior parts to get rid of the Gen 1 cup holders and whatnot. Then you have people like me that just change things to what they want regardless of the source. I'm very proud of my 2005 Ford crewcab center console that resides in my Chevy reg cab truck, for example.
If you want the car for itself and are concerned about the potential airbag replacement, inspect the engine bay and front clip area very closely for signs of a wreck repair like paint overspray, color mismatches, body bolts that the spray patterns don't match each other, metal that looks like it might have been wrinkled, ect. Any body shop guys here can give more things to look for but I've found I can usually tell when a car's been hit in the front just off this. I wouldn't go off a Carfax either as that will only grab work done by very large shops or work paid by insurance. Some maw and paw one man operation that does it for cash won't necessarily report work done to the car to a place where Carfax will get the information. Doesn't mean bad work, just not reported.
What you should be looking at, is this is your chance to make an LS that is what Lincoln should have made it. At that price it's cheap enough to rip every stupid system out of the car and put sensible systems in, like a correctly working HVAC and an engine that makes enough power to move it's 4000 pound bulk without needing a cooling system replacement every 100K miles or new coils every time you turn around.
