Requirement to start CPL training is 150h.
However, keep in mind that CPL training is around 37-45 hours of flying, whilst the requirements to undertake the exam is a minimum of 200h.
You need 250h to get a frozen ATPL. That's once you've done alll the necessary training (which does not need, but is highly recommended to have JOC).
I recommend doing the ATPL first if you're doing it here in Europe (EASA). It's what I'm doing. I miss the crap out of flying, but that way I concentrate on my ATPL exams. Remember that here it's 14 exams, not 2, like in the US. Also, that way when I get back to flying, it'll 100% of the time flying and no huge delays between flight sessions. This will keep me fresh and up to date. I recommend you do the same.
I'm told the ATPL theory is the hardest part of flight training, and I'm hoping that the peron who told me this (airline pilot) is right, because it's very tiring to do that AND a university degree at the same time.
If you do ATPL theory exams, YOU DO NOT NEED TO DO CPL THEORY. ATPL covers all the material and beyond. CPL in EU is 9 exams if I'm not mistaken (1 in the US).
Before you think about doing your ATP in the US and then converting, part of the conversion is taking those 14 exams, so it would be a waste of time and money.
I hear many people arguing about whether you should do your hour building in the US or not. I can't really contribute to that debate to be honest.
There's one thing I cans ay however. Wherever you do your hour building, MAKE IT COUNT.
That means, don't spend 90 hours doing cross countries with friends on board and pottering around in the local area. Get an instructor to fly with you every X number of hours. Have him correct you to a CPL standard. Make him give you exercises and skills to practice when you're flying alone. And try to visit as many different airspace and airfields/airports as possible.
One thing I started doing after I got my PPL was begin a Flying Diary. After every flight I would write an entry about what happened in the flights, what mistakes I made and how I think I could have avoided making them. I would also add bullet points for things I believed I needed to work on for next flight, and which problems I had successfully eliminated that I had outlined in previously entries. It had become part of my postflight routine after filling out all the logbooks.
I plan on doing this once more when I get back to flying.
There's a book, "Commercial Pilot's License", by Anneli Christian-Phillips, which gives a bunch of useful information to know about the CPL and how to be best prepared for it.
Summary (tl;dr):
if you want to go into airlines or do ATPL at all, skip CPL theory, go to straight to that
You need 150h to qualify for CPL training, but 200h to qualify for the examination
Best to do all the theory first, then fly. More efficient and better for learning retention
When you do hour build, make it count for something by learning as you gain experience.
PPL is a license to learn