CrossRoads:
Or two of these
http://www.analog.com/en/digital-to-analog-converters/da-converters/ad5308/products/product.html
$6 at digikey
Lot easier to interface with.Can't do much about DIP - not much available these days outside of DUAL DACs.
The MCP4728 is a good solution: low cost, straight forward
The AD5308 is too expensive
The only think that annoys me is that these DAC solutions can perform much better than required (8 and 12 bits instead of the required 5 bits, risetime within us -> I need 1 s ...), therefore still thinking on simpler solutions with less or low cost components.
How about the idea with fixed 32 bit buffers and shift them out at a fixed frequency?