The challenge of parasitic bulk doping in Bi-based 3D topological insulator materials is still omnipresent, especially when preparing samples by molecular beam epitaxy. Here, we present a heterostructure approach for epitaxial (Bi1-xSbx)(2)(Te1-ySey)(3) (BSTS) growth. A thin n-type Bi2Se3 (BS) layer is used as an epitaxial and electrostatic seed which drastically improves the crystalline and electronic quality and reproducibility of the sample properties. In heterostructures of BS with p-type BSTS we demonstrate intrinsic band bending effects to tune the electronic properties solely by adjusting the thickness of the respective layer. The analysis of weak antilocalization features in the magnetoconductance indicates a separation of top and bottom conduction layers with increasing BSTS thickness. By temperature- and gate-dependent transport measurements, we show that the thin BS seed layer can be completely depleted within the heterostructure and demonstrate electrostatic tuning of the bands via a back gate throughout the whole sample thickness.