In regards to backups, you could use a script or a 3rd party backup agaent (especially one with an open file agent or the ability to use VSS) on the host. For exchange you might also consider an agent based backup of the information store. Hotbackups of VMs tend to capture the state of the VM in a moment of time and are a bit like pulling the power plug on a server. You should be Ok when you restore the VM, but there's always a chance of corruption.

http://communities.vmware.com/thread/54782

http://www.dabcc.com/article.aspx?id=6489

Performance wise it would really depend on the load of active directory and exchange. In general I would avoid running an app and VMware server both on the host and they will compete for resources. You might be better off running both as VMs. If you plan for a large number of users, you might consider VMware ESX instead. It's a bit harder to learn, but it has very little overhead compared to VMware Server or MS Virtual Server so you'll get the best performance from it. It does support the setup of MS clusters as VMs, but also has a high availability option (restarts a VM if the host fails) which might be sufficient for your needs. You would also go with 2 ESX hosts with independent storage and replicate your VMs between hosts for recovery. What sort of hardware were you thinking of using and how many AD / exchange users do you need to support?