in your custom merc make sure your wisdom, (helps you gain in skills), health and marksmanship are max. Then you really dont need the other 3 skills so you can keep them really low or at 0...then use the rest to up dex to a good number cause it helps with shooting, and then the rest in agility and strength. For skills night visions is great because you fight plenty in the nighttime, weapon skill is ok, or teaching for militia is good.
as for mercs its really up to you...I've beaten the game over a dozen times on different levels with different mercs, its really preference. Obviously marksmanship is the most important so look for that, but don't go for anyone on the top 2 tiers to start off. This is mostly because they are very expensive and you really dont need a high powered merc in the early (easiest) portion of the game.
to start i like to go with wolf, ice and fox...around that...you get 2 solid mercs and a low end merc that can start training militia at the beginning.