is this a real demon?

Yeah, $1,800 is high for that car in its current condition. But, if you're looking specifically for a Demon to restore and your budget to buy the car is less than $2k there's no guarantee you'll do better. You could easily spend months looking for a cleaner car. With exception of the seats all the stuff that's missing is what most people pull and throw away anyhow (I had a hard time giving away the last /6 I had).

It looks like there's a couple small rust spots on the exterior, which is pretty common. If the frame and floors are solid spending a few hundred bucks extra now will save you a ton of time and money down the road. I've seen cars that looked a lot better from the outside need floors and frame rails. The price isn't horrible either. Sounds like most people would buy it without hesitation at $1,500. You could easily spend $300 in time and travel looking for a better one. $1k would be a steal.

I'm not saying you can't do better, people have definitely gotten better deals. But you could easily do a lot worse, saving a couple hundred bucks on the purchase to end up spending a thousand more in metal happens all the time too. I'd go over it with a fine tooth comb, if the rust damage is limited it's not a bad buy. If it needs floors and a trunk or more I'd probably pass at that price. But '71 Demons aren't the easiest to find, so if your heart is set on a demon you may have to take on some extra costs/work unless you don't mine spending some time on the search. They only made 79,959 Demons in '71, they built more than twice as many dusters (186,478!). And in '72 they only made 48,580. That's not even 1/4 the number of dusters made in '72 (228,002).