The big gotcha with 'ZI's offerings is range. Make the thing in two versions...wideband (0.1-32MHZ) and PTO/VFO replacement (5.0-5.5 or 5.0-6.0MHz)...you'll have a good solution for either application.
Don't scrimp on the rotary encoder for your DDS project. Use a Grayhill 63F256 or similar ball-bearing model. This one is rated at 300 million revolutions and turns smooth as silk.