I think the biggest bit of advice to give in all of this is to RELAX. It is completely up to you how you perform in the interview. If you want to spend money on interview prep, go for it, there are a lot of options out there and a lot of opinions on who is "the best". If you don't want to spend money on interview prep, then don't. Only you can know how YOU perform in high stress situations.... and if you're a pilot with any experience, you probably do fairly well with stressful situations.
For what it's worth, I never went through an interview prep. I simply did some research, talked with a buddy or two and went in with the mindset of "This is me. If you don't like me for me, then this isn't the company for me." meaning, I wasn't going to answer questions with what I "thought" they wanted to hear, but rather, what I really thought and felt. Guess they liked me as I've been with the company for just over a year and change. Just be yourself, be honest and most importantly RELAX. Just my $0.02