No that is not usual - it is exceptionally unusual to embark on radical treatment without a biopsy. The only time that happens really is when the PSA test and MRI show strong indicators of extensive spread, but it seems that you are not in that category? Have you been given a nurse specialist? Could you phone him / her to ask a) why you didn't have a biopsy b) why you have been recommended to have chemo?
If the local lymph node was not one that could be captured as part of the prostatectomy, they probably felt surgery was not in your best interest but they should have explained that to you :-(
If you don't have an allocated nurse, go to your GP and ask for a copy of the diagnosis / treatment plan letters from the consultant to the GP - you are entitled to copies of these so do not be put off / dissuaded.
How old are you? And are you being treated in Wales?