I would suggest that most mercenaries would consider working for an adventuring party such a different experience than what they are used to that there would really be no set standards for such a job.
Hiring a merc on this basis would be more of a roleplay encounter in my campaign rather than saying, "Okay Nigel, you drop fifty gold each for three 2nd level fighters". If there is anyone willing to do adventuring-for-hire, it would be worthwhile to figure out their motives and desires and how an adventuring career would effect them.
Most mercenaries would probably refuse the job except if the reward was exceptionally generous. However, the party could probably find a few who seek adventure and are simply biding the time with mercenary work. One could probably just as easily find qualified adventurers who are not mercenaries, especially if the party has made a name for itself.