You might have a hard time finding a developer with experience with your script. There are thousands of free scripts out there and each single PHP developer haven't messed with 1% of them all. If they did I'd be quite impressed. At least it's a popular one.
I don't have any experience with it however.
If you're concerned about the fact that not previously knowing the software would add work time, make it a flat fee job.
I never developed a full script as I can always find a free one, or a paid one that cost a lot less than the time it would need to develop it. I'm not a developer. But I do modify PHP scripts. As such, I could help you for a low price. Just PM what you need and I'll quote you.