Electronic and spin structure of the Dirac-cone-like topological surface and valence band states were studied experimentally and theoretically for topological insulator with fractional stoichiometry Bi_1.5Sb_0.5Te_1.8Se_1.2 which is considered as one of the best candidates for efficient spin-polarized current generation. By means of spin- and angle-resolved photoelectron spectroscopy we demonstrate the separation of the Dirac point from the bulk states and the helical spin structure of the Dirac cone. For the freshly cleaved surface the Fermi level is located in the bulk band gap and an exposure in residual gases shifts the Fermi level towards the bulk conduction band. Results of the theoretical calculations are in a good agreement with the experimental data. Surface morphology study shows a well-structured atomically sharp surface after cleavage. The transport measurements confirm that this topological insulator has relatively high resistance with semiconductor-like temperature dependence at low temperatures. The studied Bi_1.5Sb_0.5Te_1.8Se_1.2 crystals demonstrated a quite large Seebeck coefficient values reaching −400 μV/K at room temperature.