So this is going to be a completely stand alone voice mail system, no integration with the customer's existing Exchange environement? If so, your plan seems sound I guess. If not, you'll want to rethink this.
couple things...
You need to get a key upgrade code for your old 2.4.6 key to work on the 3.x box. If you've prebuilt your 3.0 system you should just need to run an upgrade and during that process, it'll allow you to enter your key upgrade information... just be sure to get that ahead of time.
Which TTS engine will be licensed on the key when you're done? If it's RealSpeak, you're cool, if it's TTS 3000 you might want to wax the TTS directory in the registry (HKLM\Software\ActiveVoice\TTS) before upgrading. Can save you some pain with occasional problems on TTS vendor mappings.
any languages involved?
You might want to check out the "moving unity servers" document out on the general tools page of AnswerMonkey if you want to try and move messages as well. This talks about using the Import/Export and MS EXMerge utilitlies for moving servers. Might be helpful to you if you want to do this.
Jeff Lindborg
Unity Product Architect/Answer Monkey
Cisco Systems
lindborg@cisco.com
http://www.AnswerMonkey.net (new page for Unity support tools and scripts)