Don't go by looks at all.
AGE: Get an OLDER horse. Preferrably one who is in his late teens. Why? Because the older the horse, the more riding time it's had, more handling, more time to settle into life with people = more forgiving of newbies.
--of course, there are those horses who are older and badly trained, too, but I'm talking about proven seasoned horses like typically off a ranch, ropers, trail horses, western pleasure horses
--Younger is NOT better. If you get a horse that is less than 10 years old, that horse may of been started at 2 or (better yet) 3 or 4. That means about 6 years or so of saddle time (that's assuming the horse has actually been ridden/worked for that long vs. sitting in someone's pasture). That's not a lot of time.
--Younger horses also need to know that you are the leader, you know what you're doing. Younger horses (typically, yeah, there's always the exception)...are not forgiving of mistakes as older seasoned horses tend to be.
--My first horse was 15 years old and he was the best "babysitter" I could of asked for because I knew nothing. He was very forgiving and built up my confidence. Then my second horse was a little bit more and I worked my way up to unbroke horses (I train for a living now).
--Go to your local ranches
--Lesson horses can be good too, but be aware of soundness issues
--Horse Rescues can also be great places to find horses. No, they are not there because they are messed up or whatever. They're there for whatever other reasons. I know of a rescue for example, that has and does take in horses who are already trained so well the rescue lends them out to local ranches to gather cattle and put more miles on the horse to be sure they are perfect for a new owner. Some horses even come with some basic dressage training, reining training, etc....
It's way more than lunging. If you don't know what you're doing, lunging can turn into a nightmare. I wouldn't start there. Frankly, lunging (if it's just going around in circles without any real instruction) is not good practice. But if you do it with instruction (like stopping, changing directions, changing speed, asking for softness, etc) then yeah, it's a good tool.
Get some DVDs and books:
The Revolution in Horsemanship (Amazon
Clinton Anderson DVDs
And of course, TAKE LESSONS with your horse (it's not just about riding, but overall). I specialize in teaching newbies or seasoned owners and their horses how to get along. I don't give riding lessons, I give training lessons.....from ground to saddle, how to play leader and fix/prevent problems. I suggest you find a trainer near you to take some leadership lessons.
Most of all, educate yourself through all the awesome info there is out there about Natural Horsemanship training. It's very easy stuff, just takes practice and determination. If you've got that and some patience, you're set.
Clinton Anderson, John Lyons, Pat Parelli, Chris Cox, Dennis Reis, Etc.... This is just a short list of trainers who share the same basic foundation ideas on the subject.