I agree that spending $10k on a DAC for that particular system would not be the best way to spend that kind of money. I would say that the buyer would certainly hear an improvement with a much better DAC, but, money spent elsewhere would improve the sound even more.
If the OP is planning on further improvements to the rest of the system down the road, then the purchase of a really good DAC might be viewed as a really big first step in that direction. I can see the logic in starting with really good source components first, provided that you can reasonably assess what is a really good source component when the rest of the system is not at a comparable level (a chicken and egg dilemma). But, my own preferred approach would be to find speakers that I really like, and then build the rest of the system around what works with that speaker (e.g., efficiency and level of difficulty in driving the speaker would set parameters around the kinds of amplifiers that should be in the system).