When you order your upgrade through the Product upgrade Tool online at Cisco, they will send the new Exchange Messaging Store you in your entitled software bundle. (Messaging Store, Database and Unity 5.x should be in the bundle) You can not download Exchange 2003 from Cisco.
You can download Unity 5.x.
It sounds like you are going to use voicemail only configuration.
I believe that Exchange has to be on the Failover box. I cant confirm that, but when in Failover, it will have to be loaded on the failover server. Taking Exchange off the primary server will decrease the load (processing) to the failover server which is basically doing nothing but replicating data.
Are you getting this error “Installer User Interface Mode Not Supported. The installer cannot run in this UI mode. To specify the interface mode, use the -i command-line option, followed by the UI mode identifier. The value UI mode identifiers...
The below trick might come handy when you have to add a new node to a cluster but you don't have or is unsure of the security password for the publisher. This procedure has been around for ages.
1) Login into the CLI of the Publisher.