I've just found some resistors for an R-2R ladder but they are only 1% tolerance. I was going to measure them more accurately and select some for the ladder, otherwise things start going wrong after 5 bits. Even then I think there will be too much noise on the digital outputs for full 8 bit accuracy. I'll see how well I can do.
DMA for the external memory port is quite hard to set up as it is AHB DMA - I use it in the VGA library but the code is impossible to follow 
I'll try and do a simpler example once I've got the ladder done.