Because the need for people to hear the Gospel on the mission field is so urgent, it is sometimes claimed that doing a lot of Biblical studies or earning a degree in Bible is not necessary to be a long-term missionary. “People just need the basic Gospel, and you don’t need a degree for that”, it has been said. There is a lot of truth to that statement. However, once someone becomes a Christian, you need to disciple them. And you’ll need to help new believers form themselves into a church community. And to do that, a missionary is going to need to know a LOT more than just a basic Gospel outline.
Not all types of missions work require the same level of Bible knowledge and thus it may not be necessary or advisable for all people going out as missionaries to get a Bible degree. And in many places, you can get into a country and do missionary activities without technically having a missionary visa. However, if you are going to plant churches or focus primarily on evangelism and discipleship in some sort of other ministry, then I would get all the Biblical studies (formal and informal) that you can get. If you want to plant churches and train pastors, then get an Masters of Divinity (M.Div) or an equivalent degree which is the standard for pastoral training in your home country. If not an M.Div, then a degree in theology, biblical studies, NT/OT, or similar would be most helpful.
Although I would definitely encourage those who want to purse long-term missions to go to seminary or Bible school, one great route to long-term missionary service is to go out to the mission field for one to three years to get your feet wet before going to Bible school. That way you have some idea of what you’ll be getting yourself into long-term as you start your formal Biblical studies. Your preliminary knowledge of the language and culture of the place that you hope to serve in will help you get more out of your time at seminary or Bible school, and enable you to choose paper topics that will be more directly relevant to your future ministry.
Even though the need is great on the mission field, that does not mean that less Biblical preparation is needed but rather more because missionaries need to be able to filter things down to Biblical principles, and know what is merely tradition or Western culture, and what is the Biblical truth that needs to be applied in a new setting. Although Biblical truth does not change, you can't always just cut and paste the application of that truth from back home. The church practices that you have brought from home need to be reviewed against the Bible for appropriateness in a new cultural setting. That doesn’t mean reinventing the wheel because a lot of Christian traditions from back home, you’ll end up keeping - like celebrating Christmas, for example. But some you won’t.
There are new and different questions and issues that missionaries face in evangelism and discipleship that they never met in their home country. A really good grasp of the Bible is necessary to meet new situations.
Because of age, family, or life circumstances, it is not always easy/possible for those called to long term missions to go do a full time degree but the mentality we need to have is maximalist rather than minimalist. We need to ask ourselves, "How can I best prepare myself for the ministry that God has called me to?" not "What is the minimum that I need to do in order to get a visa or meet mission agency requirements?"