As part of the interview, have the prospective reader read aloud. They may be one of the smartest in the class, but they may not be able to read aloud. Be sure to interview thoroughly. You prospective reader may not be great reading aloud, but they may be much better at describing things, such as biology, chemistry, geography etc. Some peoples schedules may not match yours, but that doesn't mean that they can't read for you.
Tape recorders are relatively inexpensive today.
Purchase one along with several dozen 60 or 90 minute tapes. You can loan your reader the tape recorder, tapes and a copy of the syllabus. Indicate that you need the material say 48-72 hours before it is due on the syllabus so that you have time to listen to the tapes. The next two suggestions are debatable, but worth bringing up. If you date your reader, be cautious.