I love my Saxon paddock boots. They aren't exceedingly expensive but I've found them to be of very good quality. I ride once a week every week and sometimes more. The only thing is there's a small scratch from when I got bucked off a horse, but otherwise the only thing wrong with them is they're dirty from me being too lazy to clean them at all
A thing to keep in mind is that if you want your wife to be surprised and not have to go try them on, you probably just want to take her regular shoe/boot size and get paddock boots and maybe a pair of half chaps that match if you know what size she takes in those. But if you don't mind the surprise being taken out of the equation take her to get fitted for some tall boots.