Tons of groundwork now but I wouldn't start under saddle until at least 3. I adopted a mustang this year and love him to pieces. He is the most gentle horse I have in my pasture. He is 3 and I plan to start him fully undersaddle in the spring. I'm holding off till then so I can work him more consistently. Hard to do right now with work and early nights.
As to where they excel - that depends on the mustang. They have the heart for anything but some have conformations and preferences that make them better at one thing or another.