You will not be able to pick the Mission Agency you have a mission from.
IMA- Impossible Misssions Agency, this agency is the best in rewards, but the hardest at times, for every mission you get at least $7,000, but have a %20 chance of coming out of it alive.
HMA- This Mission Agency is one of the best, not as high paying of danerous as IMA, but good none the less, for every mission you get a leats $5,000, and have a %50 chance of coming out of it alive.
MMA- This is the medium grade of Mission Agencys, it is low paying in that for every mission you get at least $3,000, but you have a %80 chance of coming out of it alive.
EMA- This is the easeist Mission Agency around, it is a very low payer, you get at least $1,000 for every mission, but it is also the saftest, in that you have a %98 chance of coming out of it alive.
DMA- This is the hardest and highest paying, hardest because you have a %99 chance of dying, and most high paying because you get at least $10,000 for every mission. You can only get one of these missions if you have worked for IMA.