Finding the right horse - take your time. When you find one that interests you, take another person with your for their input and if you can take him/her on a sell-back option within a certain number of days. Or take him/her on a lease with option to purchase in "x" days/weeks/months.
Costs - everyone has different costs depending on their lifestyle/barn/environment/disciplines. I keep my horse for less than $1500 a year, but I don't have to pay board, do my own farrier work, don't show and trailer very little. Other people will likely spend 3, 4, 5x that amount or far more if they are showing competitively.
Exercise - again depends on what you want to do, how much turn out is available and also depends on the horse. I can go 4 weeks without riding and then head out and my horse is fine. Other people notice a change if they miss a day or two.
How much hands on experience do you have with horses? Will you have to care for the horse all by yourself or will you be boarding?
There is a lot to consider, so just take your time. Don't rush into it and don't buy a horse on a whim or a heartbeat.