Let's assume this student is comfortable with vocabulary that occurs 50 or more times in the New Testament. This is fairly representative of many beginning Greek grammars. By 'comfortable', I mean they know the basic meaning of the word, and can recognize the various forms it can occur as.
Good reading comprehension requires very few words being unknown. Literacy research suggests that if less than 95% of words in a passage are unknown, then reading comprehension is hindered.
Using the twin criteria of 1) word frequency and 2) percentage of words in the text that are known, the GNT provides 25 nice-sized passages to work with.
This list could probably be improved if some measure of syntactic complexity were also factored in, but this should at least be a good place to start.
These passages were obtained by searching MorphGNT and the GBI syntax trees, using the SBLGNT text.