Avanti, I replied to your PM before I checked this. Same response. Like rick said you don't need an instrument rating or any formal training...just the FOI and IGI. But I'd absolutely recommend getting your paws on any instrument book you could. Knowledge is power, and the more you know the more confident your students will be that they are getting quality training. The test is identical to the instrument airplane written, with two exceptions...it's 50 questions (v. 60) and could include a question on helicopters. I took the instrument airplane, instrument instructor (also 50), and IGI at the same time...took about 1.5 hours and found that by the time I took the third exam...I had seen half the questions three times.
If you have the job lined up, go for it. It'll give you some extra cash for your own flying. Probably not a lot, but it'll help make the expenses a bit less painful.