there are so many to choose from and most work well,
decide which type, planetary(90% have), spiral, cutter, or barrel.
I have a planetary and a barrel mixer.
The barrel mixer takes a little longer, not a critical factor. The pros and cons are negliable(sp). Less headache and in the long run perhaps less expensive to go with a new one. Taking risk with a used ones, might be buying someone elses problems. Be careful there.
Any type will work, learn about them and pick the type that suits you best.
hope that helps,
Otis