NILIF is just a training method. What are you going to buy? I would only spend money on a book (like $10 - I like Pat Miller's books) or an actual trainer. Don't buy like an online course or anything. There are tons of free resources online and books you can get at the library.
What are you trying to teach them? Your success will depend on a few things:
1. How motivated your dog is (either by toys or treats or praise - you have to find out)
2. How much effort you put into it and how consistent you are
3. How smart your dog is - seriously Loki learns things in 5 minutes and I can talk to him in complete sentences and he even understands subtle things like wait, move, almost done, etc, but only because I have trained him since he was 12 weeks old. Sammy's favorite trick is to look cute and play dumb
but we love her! My point is that dogs learn at different levels just like people.
4. EXERCISE! A tired dog is a good dog!