As far as IMC goes, you would need an M1 VISA for that.
Not sure that you would. An easy test is "will you be flying with an FAA examiner?" If the answer is yes, then Visa required - there a few exceptions which also means a visa is not always required (CPL for existing PPL holder for example). A CAA IMC rating doesn't exist in the eyes of the FAA and so therefore a Visa is NOT required. You may get told by a CAA approved school in the USA that you do need a visa, but this is just them covering their arses.