IP Delivery / Cloaking project

We need to be able to present one version of site content to search engines and another for users. The two sets of content will be related but sufficiently different that users prefer their version and search engines prefer their version. We need to be able to create both versions for all pages. The system should probably work on IP identification and User-Agent identification unless there is a more accurate / reliable option. The element of the system which works on IP addresses needs to update itself as search engine spider IPs update to ensure the system works going forwards. The system needs to be easily portable so we can move it from domain/site to domain/site quickly and easily.