Culhane is the best concerning the bank of question. If you make above 90 or 95 % at their test, you will pass your ATPL.
Iatra and saron plus samra are the same course. So only one book for the 3 exams. In their book containing the bank of question, there is few page concerning the Iatra, but that is almost the same question as the saron-samra, it is just easier.
Inrat is the IR. Without IR, no ATPL.
300 euro to get a Canadian ATPL (wich gives you almost automatically the FAA one) is pretty cheap if you ask me. How much money have you spent to get your JAA frozen ATPL?
Note that you need 750 hours Total time in order to have the right to seat the saron-samra written exam. Otherwise you can only pass the Iatra. One or the other is required in a 2 crew airplane.
Good luck.