Hi,
The answer may vary according to the actual environment. Commonly, if the branch office is large enough, we may upgrade the existing stand-alone primary site to CAS and setup primary site in the branch office. Otherwise, if the client computer is not so much, we can just add MP/DP/SUP role in the branch office. As you mentioned above, the patches is download from head office in either of the hierarchy. For the detailed design consideration, we may refer to below article:
http://gerryhampsoncm.blogspot.com/2013/08/configmgr-2012-sccm-2012-design.html
Note: this is not from MS, just for your reference.
Hope the above information helps.
Alex Zhu
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