When you purchased Unity you are entitiled to the shipping version of Exchange, and you should have Exchange 2003 media in one of the envelopes.
You can install Exchange on the same server as Unity server if you are running voicemail only solution without secondary Unity server. If you have secondary server you can install Exchange on the secondary server only with Unity 5.0, with 4.2 you need a dedicated message store server.
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.