Firstly, you have to have 'commercial level knowledge' before starting a Flight Instructor (Restricted) course - there are no such animals as 'Assistant Flying Instructors' any more. Which means that, practically, you'll need to pass the CPL(A) theory exams. If you intend to upgrade to an ATPL(A) later, then you'll need both to pass the ATPL(A) exams and to keep their validity current by passing your IR(A) within 36 months of passing the exams - and then revalidating your IR(A) before the subsequent 7 years have passed.
You may not receive any remuneration for PPL instruction on SEP aeroplanes unless you have a CPL(A), although you don't need an IR(A). You may subsequently have the 'no applied instrument instruction' restriction removed from your FI rating when you have gained the required instructional experience and have completed the training requirements.
Remember that there are quite a few instructors already finding times pretty hard, so until the aviation profession picks up, there won't be too many vacancies for FI(R)s around in any case.
If you want to become an FI, you would be best advised to complete all CPL requirements first, then start your FI(R) course. But make sure that you can hold a JAR Class 1 medical before you do anything else!