If all you want to do is instruct part time, having a CPL will be no real benefit. Many small clubs would be crying out for a decent instructor who didn't want to leap off into the airlines in a year. I certainly wouldn't object to having a PPL/FI(A) as a co-worker, as long as they were good at the job and had sufficent knowledge to teach effectively.
If however you do intend to go the airline route at some point, just get the CPL done first. The groundschool is only valid for 3 years, during which time you need to complete the CPL and IR and have the paperwork submitted. Plus, it will give you sufficient differences training that you can then instruct on complex aircraft if you need to in the future.
In terms of work experience, go around some small clubs and ask in person. Many will have you along to wash aircraft, see how the ops desk is run, and are usually very friendly.