It would be possible to use a lower-end PIC32 MCU as the DCO MCU. We'll get a few advantages:
1) It can run at a higher speed so calculations will be faster
2) It may have enough ROM to store a full lookup table, or where we may only have to interpolate every second sample. As the MCU is 32bit, interpolation is a simple case of adding two adjacent samples and shifting right by one (dividing by 2).
3) Even with a lookup table, 16 bit multiplications are supported in hardware so calculations will be very fast
4) Both input and output SPIs are buffered so we need not fear losing bytes
5) It natively supports I2S so interfacing with cheap DACs from ebay is possible.
1) It may possibly be harder to write an accurate timer as it has a different architecture with more complex pipelining?
2) Price (but not an issue if using DACs from ebay).
BTW: a faster interpolation without the use of a derivative lookup table is also possible on the PIC18F if we choose one with at least 64k program memory, such as the PIC18F26k40. Chosing the PIC18F27K40 means we will not have to do any interpolation at all as it has 128k program memory.