This is a question that is as long as a piece of string.
Depends on geography, level of care, breed of horse, and many other factors.
For eg, my two cost me £1100 per month all in (shoes, feed, Livery, bedding, training, so about £650 for horse in work, £350 for semi retired mare) but I need part livery due to my job and I live in the south and I train once a fortnight because I compete.
If you had bob the cob on grass livery, you could feasibly do it for £50 p/wk all in.
For example, it costs £70 for a set of shoes every six weeks (no studs, more for studs), but this will be cheaper in other areas. £60 per month in hard feed, £50 for hay and bedding, and then grass livery / diy is anything from £40-100 per week - depending on facilities (ie top end diy with schools / Xc etc will be a lot more than a field and stable with hacking).
I don’t recommend diy for parents who are inexperienced and with an ill daughter. You need to go twice a day and do heavy lifting (poo picking fields etc) and you cannot miss a go because you are unwell or held up at work/school. This doesn’t sound like it would suit you. I used to do diy and it became too difficult to maintain in the end.
Best thing to do is enquire at local stables about their costs for part livery or assisted diy and see if it is in your budget. As I say, geography and level of care will play hugely into costs. Breed of horse too, as a good doer that is hardy and rarely needs the vet will be much cheaper than something that needs a lot of feeding or has crap feet or is a self harmer.
Then factor in feed, shoes and lessons - again ring round locally to gauge prices, or ask at your local livery yard.
Plus £600-1000 per year horse insurance, or make sure you have the capacity to spend £100-5000 per veterinary incident (£100 for basic lameness assessment and treatment, going up to £5k for kissing spine surgery or some colic surgeries).
My horses are my world but good grief I live pay cheque to pay cheque every month, live in a crap part of town, never buy new clothes, and cut my hair twice a year even though I earn £34k a year. Oh and no kids as I can’t afford them when I have four-legged ones 😂
But then I event in the south and I have two horses so I really have given myself the most expensive way of doing horses.