the best thing i recommend is first of checking the prices - you will find that the rates change from different engineers and different situations (like if they have a studio or not) or if they have an asisstnet helping them or doing the work for them

so its a good thing to ask around - another important thing is to see if they get connected to your music
a good mixer might do a good job technicly but if he is not emotional driven by your music then it will be like giving it to some who knows how to mix but doesnt bring somthing special to it- its only certain people who can mix lots of things without getting fully driven by the music those people are specific people and they get paid lots of money to deliver a great product

another thing i recommend is saving some extra money for the mixing so at least it wont cut short on the last 3 songs

and the last thing - have it mastered by a professional mastering engineer who hasnt worked on the mixing or recording of the album - somone with a fresh ear