The very best horse I've ever owned is a mustang. I bought him off his original adopters as an unhandled 3 year old stud and did all his training myself. This was almost 9 years ago.
While he was very challenging initially, he turned into a very nice horse. He never bucked or anything like that, but he did have some aggression issues due to how the adopters tried
to handle him. They never actually touched him, but you don't have to touch a horse to teach it something LOL. We got that sorted out right quick
. After that, his only real issue was extreme spookiness. Nothing out in the pasture spooked him, he didn't care about birds flushing from the brush or deer popping out of the woods or coyotes jumping out from between his front legs at a lope, but take him through town and let him see a kid on a bike or a boat on a trailer and you'd believe he thought the whole world was ending.
It took him quite a while to overcome that and while he's still a little spooky, it's never more than a big flinch anymore.
I trust this horse with my life and he's never done me wrong. I will always advocate the mustang breed to anyone with the knowledge to handle one properly.
Since the one you're looking at is already broke, then judge him the same way you would any other horse. If he's got a good mind and good training, then take him home. If he doesn't, then keep looking elsewhere.
I am concerned about this horse being "free" though. Generally speaking, even in this poor economy, a healthy horse with decent training is still worth money, regardless of breed. 9 times out of 10, a "free" horse comes with baggage; either his training leaves a lot to be desired and there are a lot of issues to work out or there is a health concern that would make caring for him expensive or difficult.
Even on a free horse, I really suggest having a trainer or experienced friend go with you to try them out...and get a PPE. Spend a couple hundred now and possibly save thousands of dollars in vet bills, hospital bills, or training bills...and lots of heartache.