"On a time and study basis there's only little difference between the CPL and ATP. So once studying, why aim for the lower-grade license?"
There is about a 45% difference in study material and BS. The ATP looks better on your resume, but it is a much more serious commitment. As an example, the minimum consolidation time (in the classroom) for the CPL is 25 hours - it is 65 for the ATP. The questions, however are just as badly worded
For the ATP, you also need to get the IR within 36 months, an MCC course and a suitable type on the licence.
For a parent with two kids and a full time job, expect about 12 months for the whole process - adjust the timescale for your circumstances. Those old Oxford books will likely be very much out of date by now. You will get notes from whatever school you sign on to.
The best advice with all flight schools is to visit them and see which ones you like.