Although I have found the system requirements for EPiServer CMS 5 on this website I can not find any detail about number of processors, RAM and storage requirements etc.
We are baselining for our 1000 person Intranet.
Can anyone point me in the right direction?
There's no hard and fast benchmarks for EPiServer as it depends on a number of factors, i.e. the number of concurrent requests and sessions that you're likely to have to support and the functionality that your pages will deliver.
With EPiServer installs I would always recommend a lot of RAM as you can leverage the caching functionality to minimise the database hits and maximise performance.
EPiServer is an ASP.NET web application that uses SQL Server, so there's a lot of information and guidance out there regarding load and performance for the platform in general.
Ultimately, the only way to be sure is to perform a proper load test to check that your planned rig will support your system and users. Given that your user base is only 1000 people I would start with a single server [with lots of RAM, ofc] and build from there.
Oh, and bear in mind your requirements for resilience and plan for disaster recovery as well - i.e. what would be the impact of a server failing, how would you go about getting a replacement in, etc.
A lot of web server farms and database clusters are justified on the basis of resilience rather than load management.