Cockatiels make wonderul pets! You're right on seeds not being a good primary diet for birds. They need variety in their diet to get the proper nutrients required for their bodies.

I feed my tiels Zupreem pellets (good brand), seeds, fresh vegetables (broccoli, carrots, celery, corn, peas, cauliflower, etc.), whole wheat bread, certain types of cereals, popcorn (without butter/salt), etc. They need variety.

Let us know when you get your bird, and what you decide on! They're wonderful, fun pets, for sure!