Should you cite a summary in the introduction that's based on information to come? If a paper starts off by saying:


  "Technology X offers a number of advantages within Y, including A, B,
  C, D and E."


do benefits A-E have to be individually sourced if they are justified later in the paper? Should I try to find a review paper that supports the claim?
I think this depends on the technology you are writing about. For some cases, the advantages might be well known (Example: computers make it easier to do calculate complicated things) and would not have to be explicitly "proven". 

If the benefits are not as perfectly clear in your field, you should definitely include a source on them. If you cannot find one which includes all of them, try to verify them indivually by providing a source or explaining why you consider them advantages (list reasons).

In case you show them to be benefits later in the paper, mention that you will do so later on. This clearly isolates them to be part of your work rather than a prerequisite.