Having completed the CAA CPL couse in the last 2 years I would advise that the JAA ATPL is a lot more difficult than passing the exams for the PPL. The PPL exams can be passed in a few weeks of study, the CPL took me 15 month on distance learning.
All depends on your level of ability, self motivation, and cash fund. Distance learning means that it yake ou longer, but it does allow you to hold down a job and obtain an income.
As advised before, go get a class one medical first as without this then you the ATPL is not achievable.
Good luck