Need help with dart purchase

It might be a bit high, but difficult to make a real judgment without seeing/hearing/driving the car.
$16-$18K would be a decent price, if it checks out okay.

Check it out thoroughly for signs of rust. Look at the frame rails, look at the channels in the trunk and around the front windshield. Look at the quarter panels from inside the trunk to see if there was any obvious sign of panel replacement.

The first rule in buying classic cars is simple. Only spend what you can afford to. Don't miss a mortgage payment because you bought a Dart.
So, evaluate your finances and determine what your comfort level is. Since this is a dealership, they are gonna stick you with extra taxes and fees, so factor that in.

Also, that is the "asking" price. Find ways to de-value the car and get some money taken off that price. For instance, the car has an E-body console in it with a T-handle shifter from a 71-74 Challenger. I don't think that is the correct side stripe for a 76 Dart. The engine bay is painted black, instead of body color. The tail panel is blacked out, I think it's supposed to be body color.
These things don't make it a bad car, but you can use them to negotiate. Like I said, find a way to de-value the car.