I no longer have horses.  I ALWAYS picked for disposition first, decent sloping shoulder next, and I personally want a horse with brains since with my MS sometimes the horse has to be in charge.  I don't want the horse to obey a stupid order, and when I am tired I can't control my body too well.

Nice, friendly dispositions make up for a lot of faults.  Remember you want a horse to have fun with, don't overmount yourself, this is the biggest mistake people make (just like I did at first, luckily I looked for a good disposition also!)  I have a pet peeve of people riding horses that are too tall or too wide for them.  If you don't already know how to train get one with at least some training,since if you can't just get up on the horse and ride he won't be anywhere near as fun for you.

Luckily for you there are lots of good horses out there cheap, many with at least some training.  Get the opinion of a good horseperson, they may notice problems you don't see.  Get a vet check too and if you can afford it I would recomment medical insurance for the horse--emergency surgeries are EXPENSIVE, and be sure to tell the vet that the exam is also for the insurance purposes.

Good luck in finding your perfect horse!  Just realize YOUR perfect horse might not be perfect for someone else. 

Get a good dispositioned horse with comfortable gaits that fits you.  Easy?--not!  But don't give up, and if you are lucky your horse will pick you.