Dr. David Livingstone wrote on qualifications of missionaries: (in 1882 Regions Beyond magazine)

Missionaries ought to be highly qualified in every respect; good education, good sense, and good temper are indispensable. ... A sound mind and a sound body, independence of character, strength of judgment, aptitude both to learn and to teach, are of great consequence. An ability to acquire and retain languages; tact in managing others, so as to conciliate and yet to retain proper dignity and self-respect are of great importance. There should also be an intrepid spirit of enterprise, decision, cool courage to meet sudden emergencies and to overcome dangers, gentleness, powers of endurance, and temperance. We want our best, most able, and greatest men to do the highest and most important of all work, making known Christ's gospel where it has not been hitherto heard.

Biblically, the qualifications for missionary service would be at least parallel to those of "elders" and "deacons" in 1 Timothy 3 and Titus 1. Certainly, their ministry skills and doctrinal convictions must be tested and approved in the context of the local church. A missionary should have experience in a good church is he/she is to be involved in planting a good church in another culture. When we think of how much time and effort it took Paul to be approved and sent out, even after extensive training in the Scriptures in the highest level of Jewish education, you shouldn't be in a rush to get to the field. 

Foundational qualifications for any missionaries include:

An intimate love for God as exhibited in a gospel-driven relationship with Christ and lifestyle

A lifestyle and character congruent with those describing elders and deacons in I Timothy 3 (including descriptions of how to relate to spouses and children, if applicable)

A set of skills and gifting that align with the proposed ministry on the field, as observed and confirmed by the local church

A mature understanding of the Bible, and the ability to handle and apply its direct commands and principles to people’s lives and ministry settings A proven ability to work well in team settings and submit to leadership

Theologically driven convictions about the why of the mission (ultimately for God’s glory) and the methods for mission.