Are you intending to keep the horse alone? I would strongly advise against this. Horses are herd animals and many are very unhappy being kept alone. If she has been used to being in a riding school with lots of other horses around at all times, she may become very distressed at being kept on her own and become difficult to do, or dangerous, or potentially injure herself in the field.
As you want to hack her, hack her alone a few times before buying, and see what she is like on her own, most horses are quite different on their own instead of with a group.
How much do you know about stable management? Ideally, it would be best to keep your first horse somewhere where you can get advice on issues such as healthcare, feeding and shoeing.
Are you confident tacking up and grooming alone?
Are you confident catching from the feild and turning out?
How would you deal with any problems?
Do you know how to recognise signs of ill health in a horse and when to call a vet/dentist/farrier?
What do you know about worming and feeding?
I know it sounds like I am being negative, but I have seen a lot of people get into trouble when they buy a horse to keep on it's own when they only have minimal experience of how to care for one. It would be so much better if you could start off keeping your horse at the riding school or another livery yard.
Ex riding school horses can change a lot when they go from being in hard work to only being ridden once or twice a week, but equally they can be fine.
Despite the fact you obviously trust the riding school, make sure you get a full vet, and if possible, get someone experienced but unconected to the riding school (do you have any horsey friends?) to come and have a look at her.
If she is worth it depends on the price. I wouldn't pay more than £1000, maybe a bit more if she came with everything you need, such as rugs, and tack as this is obviously worth money in itself. You could easily get 5+ years out of her, but equally if she has always been in hard work, she may not make it to 20 without serious problems.