You can do this fine. I have designed a couple hub and spoke Unity designs. Unity is at the hub, next to an Exchange server. The remote sites vary from 5 subscribers to 100. You just need the bandwidth capacity to handle CallHandlers and voicemails. If you are using UM, it's better because everyone will stream out of Exchange, rather than using Unity port and TSP. (Outlook/Exchange has the wav file, they use media player or the builtin outlook snapin to play the file)
If the remote user picks up the phone to check VM, then you need the QOS and bandwidth for this to happen. Design your WAN accordingly and you should be fine.
Have the secondary Exchange server is not a big deal. It is a little odd for Unity to have this because it's offsite. Cisco normally does not recommend this because the time required for MWI services, etc to read the Exchange databases for new/read messages. If the roundtrip times are slow for some reason, the MWI may or may not light the lamp. Sounds like your bandwidth is good, but still provision it accordingly for voice.
Create your Exchange servers and Site links. This will cut down on replications in AD, etc.
Hope this helps.
