You're definitely missing a bunch of stuff! You've got a standard rear seat car, so not a fold down seat which should make some stuff a little easier to find. Back seat area should look like this
Looks like you're missing the headliner, door panels, package tray liner and rear sail panels. The plastic rear sail panels will be the hard part, they get brittle and because they're large pieces they're almost all broken. Here's a link to a set on eBay, this guy is asking crazy prices but typically if they're in good shape they do go for a couple hundred bucks, and they're darn near impossible to ship.
1973 1974 1975 Mopar Plymouth Duster Interior Sail Panels | eBay
The front should look about like this car. This is actually a Dart, but the door panel layout was the same for Duster's and Dart's 73+. Looks like you're missing the dash pad, ash tray door, the upper door pieces (plastic), door panels, handle pull cups, and armrests. It also looks like that was an A/C car, so the vent under the dash in this picture is missing as well.
You can probably find better pictures on the internet but that will get you started. The door panel patterns were different from year to year and by model but for the most part 73+ Duster parts will all interchange. Some of the Dart stuff will too, the dash pad, ashtray and front door panel stuff all will if you're not worried about the patterns matching or being correct.
