Low temperature scanning tunneling microscopy is used to image the doped topological insulator . Interstitial Fe defects allow the detection of quasiparticle interference, and the reconstruction of the empty state band structure. Quantitative comparison between measured data and density functional theory calculations reveals the unexpected coexistence of quantum well states (QWSs) with topological surface states (TSSs) on the atomically clean surface of . Spectroscopic measurements quantify the breakdown of linear dispersion due to hexagonal warping. Nonetheless, both QWSs and TSSs remain spin polarized and protected from backscattering to almost 1 eV above the Dirac point, suggesting their utility for spin-based applications.