As a rule, budgies shouldn't get a pellet only diet. They are seed eaters by nature; wild budgies eat a lot of millet (albiet a different species than the domestic millet we feed our pet budgies).
Budgie seed mixes are low in fat, anyway.
In my experience, budgies don't really care for fruit. Most like veggies and grain-based things like bread, cooked pasta, rice, ect. Leafy greens are really good for them.
They can have some pellets, but budgies can be picky about them.
For seed mixed, most budgie mixes are pretty similar in the seeds - red and white millet, canary seed, oats, and flaxseed. "Fancier" mixes often include a lot of dried fruits and veggies that your budgie will never touch - especially if they are getting fresh veggies. I like the Volkman parakeet seed since it is a nice, relatively plain seed mix. It's not a complete diet (no seed mix is) but with veggies and other healthy fresh foods, it seems to be a good diet.