The typical cost of the horse for general family use is the cheap part, £1000-4000 the up keep ongoing monthly costs livery etc.. 200/300 a month depending on if you have full livery or DIY etc....... big commitment. easier of course if you have room on your own land for a stable/ paddock.
A good way to start is to go to some local livery stables local in you area, most have riding schools attched, most will know a horse dealer or can advise further or maybe assist in locating the type of horse, size, ability (jumping, showing, or just a family hack ?) .
If you sell on the horse a year or two down the line,if the family outgrow a pony etc then its typical you should get your money back, but the cost as I said is the small part as its an ongoing expense once you have it.
make sure the family if they are not experienced riders get a horse that is bomb proof. Cost will vary depending on how far the horse is broken in etc how old 5/8 year old should be o.k. ? Make sure the horse is also very o.k. at having his feet lifted/ picked out. and not skatty on that front. Its not fun worrying about half a ton of thrust coming your way when your thinking look here horsey im only cleaning your hoofs. 🙂
Get public liability insurance for hacking in public, insure the horse and look into vet insurance.
Also when you locate the horse, espeically if through a dealer or school ask for a few weeks trial,whereby you and the family will interact ,look after and ride it (maybe at the local horse school) , they should be helpful towards you.
Good luck , enjoy.
Oh if you yourself have a lesson at the school make sure the farmers not outside and firing up his chainsaw. Horse's sometimes dont react too well to chainsaws. 🙂