Be worth doing a business requirements spec too. Infrastucture BRS's are small in my experience, but its good to get a clear list of business requirements before going into the project. Make sure they are actual business requirements and not technical requirements. Modelling the deployment process as part of this is probably useful too - again make it a business process map, not a technical map. Shouldn't take you too long and you'll be surprised at what you uncover.

Kimbo

 I use BPMN. It'll do the job. Not really a template. Just include a couple of sentences about each activity and define your actors in a couple of sentences too. Triggers, etc will be on the diagram.

Still think you should do a set of business requirements too. Specially as it sounds like the business has already made a lot of assumptions. If nothing else, it'll protect you if it all goes belly up. Good luck with it.

Kimbo