single engine IR is not transferable across to a Multi engine IR
I don't believe that is correct. You have to convert to multi IR but the conversion is about 5 hours of flying, I believe (and do a twin rating as well of course).
I found the IMC a tremendous way of improving my general flying skills. However, if you are going commercial you need to decide when and whether it is worth doing the IMCR. To do the IR you need to have done the exams first. You can either do the stand alone IR exams, of which there are 7, or do the full ATPL course with 14 exams if you are going straight on to CPL and IR. Remember that you have 36 months from the passing of the last exam to complete the CPL and IR, so don't do them unless you are sure you want to go ahead.