Unless you're planning to breed, gender isn't really as important as temperament. Yes, temperament can be influenced by gender (as you've read, above); but "mind" is really what you're looking for rather than, well, reproductive parts.
As usandpets mentioned, breed can be a factor in temperament, but it isn't an absolute determinant. Horses, like people, are individuals and each has its own "personality" (or is it "horse-ality?"). ;) Confusing? Absolutely. Welcome to the world of horse-shopping!
That said, my "termperament shopping list" was narrowed down by (but not limited to) the following:
(1) Breed - What are you planning to do with the horse? Which breeds fit best within this discipline? Of those breeds, which tend to be hot? Which tend to be calmer?
(2) Experience - Has the horse been trained for the discipline I'm considering? Does it have experience in the show ring, on the trail, etc.? If it has experience in "x," then when I do "x" with it, it may be calmer because "x" won't seem so new to this horse.
(3) AGE - In my experience, older horses tend to be a little more experienced and settled. Not always true, but a good rule of thumb. When I was looking, the age range I preferred was 11-16 yrs.
And then --
(4) Gender - I prefer mares, as well. Just my experience. I've had two nutty geldings, but my mares have always been great. Another thing: I prefer mares that have foaled over maiden mares. I feel that mares that have foaled tend to be more settled. (I know that having children certainly settled me, hormonally-speaking.)
After that, there are a number of other factors that will go into a final decision. I won't list them here because we're talking about temperament factors.
Take your time. Ride plenty of horses before settling on one. (I rode several before and several more after the horse that I finally bought. I knew she was "the one" because I compared all the other horses to her. She was my touch-stone.) And most of all: Don't let anyone talk you into buying a horse that isn't quite right for you, right now. I've made more mistakes by allowing someone to massage my ego: "I've seen you ride. You're good enough to handle a little more horse than that beginner-safe one I just showed you. THIS horse is one that you can 'grow into!'" BIG MISTAKE.
If you're looking for "safe" and "confidence-builder," get a good-for-me-now horse.