What i am looking to do with multisites is this:

1. Have a master site which acts as a repository for all the content (articles, menus, contacts, modules extensions etc).

2. Have many slave sites which will get there content from the master site. The only thing different will be the theme (template + media)

3. there will be regular updates to the content on the master site which I would then want to filter automatically down to the slave sites ( update in only one place which leads to content in slaves being updated)

4. I need to be able to check statistics on all the slave sites seperately

Firstly, I take it this is possible with multisites? If so what do I need to buy?
Secondly how would you recommend structuring it ie. slaves on same database, slaves with their own databases with some shared items?

Many thanks

1) Use the master as repository of extension is OK.
Concerning the content of the extension that you would like to share with your subsites (or slave sites), this is only possible with a limited number of extension except if you decide to share the whole website content.

2) Does that mean that slave sites will have both their own content and also shared content coming from the master.
The word content is too generic and must be qualified with the name of each extenion content you would like to share.
Jms Multisite can only share a limited number of extensions content.
See the list of extension that can currently be shared.
www.jms2win.com/en/list-of-extensions?vi...id=269&Itemid=53

If you want to share the whole content except the "/templates" directory and the "/media" directory, this is effectivelly possible.

3) Again here, is which content (for which extension).
If you make reference to the section, categories and articles content, it is possible to read the content from other website when using the "Article Sharing for JMS" extension. See its demo that explain how it works.
tutorial.jms2win.com/joomla/multisite-12...ingforjms/index.html

4) For statistic, this depends on the tool that you would like to use.
If you are using your hosting provider statistic tools, in general, you can have statistics by domain name.
If you want to have specific Google Analytics statistics, this is also possible.
For example you can find a "multisites google analytics" free module that allows you enter specific code for each domains.