Advice on buying a '75 dart.

Here's my 2 cents, for what it's worth.
If the car is worth 6 grand to you, then buy it.
The car looks to be in good shape according to the pictures you posted. (thanks for that!)
I don't like the interior, but it looks like it fits well and would be comfortable.
I would check on the underside where the front seats are mounted to the floor (from the inside under the carpet) to see if there are reinforcing plates on there, they do need them to be safe. Other than that, plan on finding some original seats and have them reupholstered.
Look for rust or holes in the torsion bar cross member, front frame rails especially under where the upper control arm mounts, inner fenders, rear frame rails, trunk floor, and floor pans.
The body looks good from the pics, but use a fridge magnet to check for bondo especially in the bottoms of the rear quarters, rocker panels, and bottoms of the front fenders.
Check closely at the front of the roof where it meets the windshield trim for swelling or softness when you push on it.
These are all of the usual rust out places on these cars.
The engine would probably be good seeing that it's a slant 6, but listen for any unusual noises or smoking while it's running as well as oil or water leaks.
Drive the car and make sure the car steers and stops the way it should without clunks or wandering.
The car most likely has a 7 1/4 rear end in it and looks to be big bolt pattern from looking at the wheels and would have disc brakes which would be a plus.

I think that 6 grand is top dollar for this car IF everything checks out okay.
Don't look at it as an investment, because the more money and work you put in it, you will not recover.
I am really into these years of A-Bodies and have owned several of them (still do), and they just don't bring big bucks and are generally hard to resell, but are fun to own and modify.........

BTW, the prospector emblem in the grille looks to be off of a '80s Dodge pickup, it's not original to the car!

I hope this helps.