Expressway must have reasonably high-speed/low-latency access to all CUCM/IM&P/CUC cluster nodes as it will be proxying traffic for users to assigned to all of them (e.g. IM&P sub-cluster node a user is assigned to or CUCM Call Control Group node in CSF device Device Pool).
Without knowing extenuating circumstances, I would say that if the CUCM cluster meets the bandwidth/latency/jitter/loss requirements from the SRND and the Expressway-C/-E pair are located along with the publisher then your WAN should be fine. Just be aware that Expressway clustering has a FAR stricter set of requirements than CUCM does so if you plan on splitting that up be sure you're <10ms one-way delay.
