The medical is not an issue if you only want to instruct. You will need the CPL ground exams. The IMC is well worth doing and as 10 hours IF training is a pre-requistite for the FI Course then its almost essential. You will need 200 hours total with 150 PIC to start a FI course. You will also need a 300 nm X-Cty
Go for it, your engineering experience will stand you in good stead.