If you have the time and money available to undertake the instrument rating and commercial in one go then i would strongly aadvise a part 141 school. This will combine structured flight and ground training and allows you to get the commercial in less hours than doing a straight part 61 course (190 hours against 250 hours i think). It is a little like the difference between integrated and modular in the UK but doesnt work in nearly the same way. As for the FAA ATP, well that's irrelevant. There is only a minimal amount of training required and can only be completed after 1500 flying hours are achieved. Good luck!