We just bought a new horse for hubby and are currently looking for one for my daughter so this is pretty fresh with me.
Some are to do and others are to ask.
Has she had any injuries, past or present? Diseases? Foundered in the past? What type and size bit does she use? How is she for farrier? Shoes or natural trims? How often? Baths? Standing tied? Does she have any bad habits? Does she pull back? Will she stand for a mounting block? What disicipline does she ride? Does she like treats? (that one is very important! Lol!) What type of wormer, paste or daily? Is she used to being stalled or pastured?
What BRAND of feed does she use and how much? (you may have to switch her over to the brand you normally use.) How many feedings a day is she used to?
When you see her, see how easily she will pick up her feet for you. Catch and saddle her yourself. Bring your saddle to see if it will fit. Be sure to ride her first in an enclosed area. Unless you are a VERY experienced rider. (and wear a helmet!) Be sure to have a vet check. On a horse this age, I would probably also include xrays.
This is all I can think of off the top of my head. If I think of anymore, I'll add them.