It's perfectly fine to buy trees that are already fruiting and of course it ensures you know what you're buying so little/no risk of mislabelling. A larger tree will spend most of its resources establishing the roots before growing out, and it might abort some of its fruit, but it'd still be way ahead of where a younger tree would be. Fruit and flowers do slow growth but that's true of all trees, not just recently-planted ones. Some people like to remove all flower buds the first few years to maximize growth of the plant, but that requires more patience than I have.