End of year 7? So you've got two years until the GCSE mill starts, if you decide to go down the X exams in 2 years route like the schools do.
I'm worried she will take the pee and refuse to actually do any work
She will, and she totally should, while she is "de schooling". The rule of thumb is that you allow a month of deschooling for every year the child has spent in school, assuming no trauma occurred in school. That means at least until Christmas, and maybe even until next Easter before you start worrying about whether she is doing enough "work". Instead, you help her pursue her interests - the things she enjoys doing, is good at, feels comfortable doing. Get yourself networked with local home educators - take her to groups, informal meet ups (she might well function socially really well with rather younger children, and the joy of Home Ed is that that's considered completely normal and unremarkable. One of my children has a best friend 18 months younger than them; another has close friends 3, 4 and 5 years younger. Normal normal normal).
If she says she wants to learn to knit, help her learn. If she wants to read a book, help her if she needs. If she wants to plant some flowers and watch them grow, facilitate it. At the end of the deschooling period, you are likely to look at her and see a child who is happy, curious, inquisitive, independent and self-reliant, and infinitely more confident than she is now. You may then decide to introduce more formal learning, but you can do it in collaboration with your child, and also in tandem with helping her develop all sorts of other skills (like, being able to navigate shops and money and change; being able to bike, skate, swim, climb trees, light a fire; whatever)
I'm a single parent who works mostly from home.
Just be ready for a tough month or even two months while you both settle into how to make it work. 10 minutes of focused 1-to-1 attention can fill a child up for another half an hour where you can get on with something yourself. But it may take a few weeks of her being really clingy and anxious as the immediate school recovery stuff happens. Call in favours from friends and family to give you a break. As you get to know home edders, you'll gradually find that she is confident around other adults and that gives you opportunities for a break too (honestly, a casual observer would probably get completely the wrong end of the stick about which children belong to which adults if they watched our home ed group about its business, and then all heading off "home" afterwards :) )
I'm worried I won't be able to teach her to the required level to pass her exams which I do want her to do It is not time to worry about that. Just bookmark this page and, in 4 years time, come back and look at it, and have a little fond laugh at how clueless you were about how home education works. When it comes to it, you'll get the syllabuses you need, and the teaching notes and work books, and you'll get onto the Home Ed yahoo groups for people doing structured learning and preparing for exams. And you'll find out that there are often little groups of HE teens doing exams together, or going to college for some GCSEs, or learning everything they need to about chemistry by doing internet research and collaring Aunty sue's husband's second cousin who is an industrial chemist (but not now - you really really have to deschool first or you'll get yourselves into a right pickle with a resistant child and stressed out mum)
I'm worried mostly about messing it up and her going into year nine further behind than she is . You really really think that you - knowing, and loving, and listening to your child, with time to devote to her one-to-one and concentrate on how to help her be happy and productive - you can do worse than the school has? Really?
One of the biggest things about home ed is that there IS no "behind". Children learn at their own pace, because HE life is not a race. I have watched one of my children make huge strides in physical co-ordination recently (see what I did there? heh). It's later than some other children gain those particular skills, but because it is happening at the moment that this child is ready cognitively and physically, and is receptive to opportunities to develop those skills, they are improving ridiculously quickly. With no external adult coaching, just being around other friends who are slightly further on with these skills than my child is. Children - all children - are brilliant at learning. We just have to provide the right environment, the right resources, the right company, and back the hell off and let them get on with it. School really interferes with that process, especially when a child is battling with special needs of one kind or another at the same time.