getting a car

The car says Deamond on the back, it looks like a Duster but it is different. Dad said it had the big six motor.
I'm pretty sure that it says ''Demon'' on the back not ''Deamond''.
On the front fender I can see the ''Demon'' decal as well.

A Duster and Demon are basically the same cars, other than these differences:

They made Demons in '71 and '72 only, and are the same body as a Duster with some changes.
The tail light panel is different and so are the tail lights which are specific to the Demon only.
As for the front end, all of the sheet metal that is unboltable (which is the fenders, hood, bumper and brackets, lower valence, hood latch brackets and support, and the entire grille assembly) were the same as a Dodge Dart of the same year. It is commonly referred to as the ''front clip''.
The name plates on the inside of the upper doors which are a rectangular black plastic say ''Demon'' on them instead of ''Duster.
There is also a different style of side and tail panel stripes than the Duster.
And of course, the Demon decals on the front fenders and rear panel.

Your V.I.N. which is located on the dash on the driver's side visible through the windshield would be as follows:
LL29C1XXXXXXX if it is a 225 slant six car.

You have a desirable car there, a lot of people want to restore them, especially the '71s.

One concern that I have from seeing the picture, is that the car appears to have a factory vinyl roof that is peeling back, and under it there seems to be significant rust.
This is a common problem with vinyl top cars because the factory put little to no paint on the roofs of cars that were destined to be equipped with the vinyl roof option to save money on paint.
Then over time, moisture gets under the roof between the metal and vinyl and starts to rust.
It can be a big job to replace the entire roof, or if it's just the skin, it's not too bad........just time consuming.
And then you have to inspect the underside of the car closely for rust and holes, and if the frames, floors, inner fenders, rocker panels, and exterior sheet metal as well as the roof is bad on this car, it might not be worthwhile to save it..........and that's the sad truth.

Do a lot of research and reading on here, there are many posts and detailed info. on doing major rust repair to these cars.

You did the right thing first by joining up here and asking questions!
There are a lot of great people on here that are willing to help you, I bet there are some close to you that could look the car over with you and be honest about the general condition...........