Such amazing and valuable advice, thank you all so much for taking the time to get back to me!
It's not a story I've fallen in love with, I think it needs quite a bit of work and there are some big holes in it, so for me, I was only considering it on the value of having experience and actually creating a book, but I think you're right when you say I'd be better spending the time making my own project to use in a portfolio etc. The ultimate goal for me would be an agency as I really have a hard time negotiating and justifying my worth, I'd be better off putting my time into getting representation and working on my skills.
I think I will certainly lay out what needs doing in a quote as a reply to the time involved. I know before I really started learning about the industry that children's books seemed like a very simple job - how wrong I was! I find simplifying harder than realism, it's been a huge learning curve but I absolutely love it and without a doubt want this to be what I do for the rest of my life.
Again thank you all for taking the time to respond. I'm making my way through SVS courses bit by bit, not even attempted to think about the money side yet so I really had no idea of the going rate, but I know roughly how much work it will take and that's why red flags were waving over the budget. If it would take me a week to do, start to finish then fine, but I don't think that's going to be possible.
So glad I've found such a lovely, welcoming and helpful forum! I have yet to make any arty friends so it's wonderful to find like minded people here!