Well, MORB's thread is getting rather long...
Rather than specify up front what item you'll get, why doesn't a special trade window open on completion of a mission. This would be populated with a random selection of reward items (maybe 6-12 in total) from which you could pick exactly one. Maybe a selector could be added to the mission window that allowed you to 'skew' the type of item generated, so you could say 'I am looking for melee weapons' or 'I am looking for armour' and there would be a number of the possible rewards of that type (say about 1/3 to 1/2 of them).
For team missions, everyone on a team would get such a trade window with their own selection, but in team missions, there should be no way to 'skew' the results, since there will be many more items to choose from in effect.
Or the window could be empty to begin with, and have the selector built-in to it, and once a category was selected and a button pushed, the contents would be generated and the selector and button disabled. Then the same mechanism could be used for both solo and team missions.