I believe you need the CPL exams to become an FI to teach the PPL. To be paid do you require a CPL too?
Yes and No. A FI with a PPL may be remunerated for Flight Instruction and Examining
If you were to go on to train pilots to become FIs what would the process be?
Under the UK National system a FIC Instructor needed around 2500 hours instructional experience and had to undergoe a rigourous selection process however; EASA has seen fit to lower the standard by about 80%
FCL.905.FI FI — Privileges and conditions
The privileges of an FI are to conduct flight instruction for the issue, revalidation or renewal of:
(i) an FI, IRI, CRI, STI or MI certificate provided that the FI has:
(1) completed at least:
(i) in the case of an FI(S), at least 50 hours or 150 launches of flight instruction on sailplanes;
(ii) in the case of an FI(B), at least 50 hours or 50 take-offs of flight instruction on balloons;
(iii) in all other cases, 500 hours of flight instruction in the appropriate aircraft category;
(2) passed an assessment of competence in accordance with FCL.935 in the appropriate aircraft category to demonstrate to a Flight Instructor Examiner (FIE) the ability to instruct for the FI certificate;
Just because you have the hours in (1)(iii) does not mean you will be able to achieve (2) as easily. but that depends on which State you are in!