markham,
Yes you can instruct on a PPL, but you need the following:
1. 300 hours total time,
2. To have demonstrated the knowledge level of a CPL (ie. take the CPL exams).
3. Complete an FI(R) course.
I would suggest that all of the above requirements are aimed at those recreational flyers who want to instruct and pass on their experience rather than gain hours or get paid. The cost of setting out on achieving all of the above is more than going through the modular route of obtaining a CPL with FI(R).
In effect if you were a seasoned flyer with 300 hours already in the bag then yes it is worth it, but in oyur position I would suggest not.
Regards
Obs cop