Before I adopt a child, is there a way to test a 2 year old for ADHD or other brain problems with a cat scan, mri, or any other way?

Probably not. Age 2 is far too young to be able to test for many of the disorders of concern to you, and scans are not going to help. Infant psychologists can estimate overall intelligence and functioning at this age, but that may be affected by the child's circumstances before adoption. They can pick up current problems but are not apt to be great at predicting future ones such as adhd.
Avoiding problems. The problem with predicting our nervous system is it takes so long for it to mature,about 22 yrs in females/25 in males.Like a book,published only pages at a time, we don't get to race to the middle or last pages to find out what happens.Kids mature over time and many of the problems you fear cannot be detected until they emerge with age.You need to be ready for joy or hardship,no guarantees.
Social workers have. To take a medical & family history when they remove a child from home. A history of exposure to alcohol, tobacco or drugs, family history of physical/mental illness &/or neglect/abuse may allow you a "special needs adoption" to access funds for potential future needs. Evaluations by a developmental/behavioral pediatrician & early intervention define current functioning, but are not predictive.
Not that I know of. Certainly not ct or mri. You may be able to get a neuropsychologic evaluation. Problems with adopted children often take a while to surface.