I read somewhere that if you install the IF-2000 board in place of the DMU scope board, you keep all the the functions of the DMU except for the RF scope function.
That is my understanding as well. I currently have a DMU200 and have been contemplating the same thing you are. I'm having a tough time with the decision because by the time you buy the IF-2000 and the other stuff you need to setup a panadapter, you are in it for around $500-$800 depending upon what route you go.
The other thing seems to be that with a DIY panadapter solution, you are limited to a 192khz view of spectrum, whereas with the DMU, you can go up to 500khz (maybe higher, I can't remember), therefore you can see the entire band on the display.