I was considering developing an application which would:
1) form part of a suite of similar applications; and 2) allow the user to download additional chargeable content.
I would be extremely grateful if somebody could point me in the direction of any resources where this approach is discussed programatically and practically (i.e. Apple's requirements for doing so as well as code samples outlining the appproach).
I think what you have in mind is In App purchases, you can start with the in app purchase guide from Apple and the quick start guide. I've implemented this and found it very effective and fairly simple to implement.
Programmatically it uses the StoreKit framework which is described in the guide I posted above.
If you want to address the suite of apps you can also run ads that cross promote them, I've seen admob used for this it has an in-house ad feature that allows you to serve ads for your own applications for free across your own apps.
You can charge for additional content with In App Purchases. After reading Apple's In App Purchase Guide as recommended by paulthenerd, you can also check out ilime.com and urbanairship.com. These are services which make In App Purchases easier to implement.