What can of Duster can I get for 23K

For a 1970 or 1971 Duster 340 - you probably need to double your budget if you want a car that is already done / restored at least at an amateur level.

Here is one currently For Sale on this site (Take note of work needed):

You would be fortunate - maybe lucky - to find a decent 1970/71 318 Duster for $23,000 - that is the market today for a V8 equipped 1970/71 Duster.

If you could adjust your expectations you possibly could find a very nice 1973/74 Duster with either a 340 or 360 - for a number approaching the 23K budget.
Last spring a really nice 1973 triple black (Vinyl roof) 340 4 speed Dart Sport which was garaged for most of it's life sold for about $25K and it was easily worth the price.

Earlier year Darts are available but personally inspect --- the rear wheel wells are not as accommodating for larger wheels / tires.

Good luck with whatever you decide.