The advanced scouting budget results a variance between what a player's "true projections" are, and what you see as the player's projection. As shobob points out, even a $20m AS budget will not show you 100% accurate projections, but $20m will be as close as you can get. The further away from $20m you get, the more variance there may be from what you see and what is reality. $14m seems to be generally regarded as OK.
For IFA budgets, projections work exactly as descibed above. But the higher the IFA budget, the more IFA's you will see. As shobob also points out, there are no cutoffs based on ratings. It appears to be a random thing as to whether or not you see a particular IFA. I do not believe that $20m IFA budget insures that you will see every IFA, but you will see most of them. The further away from $20m you get, the fewer IFA's you will see. But there is still a chance that you can see a stud IFA with, say, a $6m IFA budget that another owner with an $18m IFA budget will not see. But the projections you see may be way off-base and you should have a much less level of confidence in them.