1) If a client doesn't want to buy any hardware appliances (eg SCG) is it possible to buy a cloud based hosted SCG? Is SAM essentially a cloud based hosted SCG?
SAM is not an hosted SCG. It is basically cloud-based WISPR solution, which can be used with hosted SCG or local ZD (actually if you order license to be used without local ZD, license includes license for hosted SCG use).
2) If a client purchases a SAM license, will he be also required to purchase anything else? (aside purchasing extra licenses for additional AP support).
No, you don't need anything else. This is the point, all you need in one package -- it is User authentication solution, wlan and AP management and support in one package. Only if you want something additional, you need to buy additional services, such as Location services or SCI analytics.
If ordering, be sure to order proper license - including hosted SCG services or to be used with local ZD, they have different part numbers and price, but names are very similar.
3) Does SAM/SCG/vSCG have ALL the features of a standard ZoneDirector appliance? (for example, Channelfly, background scanning, WIPS etc).
No, SCG has not all features of ZD, but the most essential are there. You get much more scalability in SCG, but it doesn't support all features ZD has.
Hope it heps...