First the good news.......Yes you can instruct part time. Indeed many people do.
Now the bad news.........First after finishing your PPL you will need to complete either the ATPL or CPL exams. You will need to build 100hrs P1, then do the CPL course and FI course or build 150hrs P1 and then do the FI course. If you choose the later option you cannot be paid for instruction as you will have no CPL.