The advantage of jelly rolls is that you get a lot of related strips of different colors at once. Not all brands are evenly cut, however and you can't prewash without a lot of trouble. IMHO, FQs (fat quarters, 18x22") are a better bet for quick variety. I recently went to JoAnns and bought 20 1/4 or 1/2 yard pieces for a crazy quilt. It took forever but was worth it to me. Jelly rolls are a marketing tool and may not be the best way for you to go. I recently ordered 4 'layer cake" packs (10" square pieces) which were perfect for making 8 4" half square triangle units from pairs of fabrics. (although I did not prewash, a definite disadvantage)
IMHO, you are better off deciding on your pattern, then working from there for your fabric needs.
many places have patterns rated by difficulty. My teacher recommended
www.quilterscache.com.