Paramagnetic substitutional carbon (C[B],C[N]) defects in hexagonal boron nitride are discussed as candidates for quantum bits. Their identification and suitability are approached by means of photoluminescence (PL), charge transitions, electron paramagnetic resonance, and optically detected magnetic resonance (ODMR) spectra. Several clear trends in these are revealed by means of an efficient plane wave periodic supercell ab initio density functional theory approach. In particular, this yields insight into the role of the separation between C[B] and C[N]. In most of the cases, the charge transition between the neutral and a singly charged ground state of a defect is predicted to be experimentally accessible, since the charge transition level position lies within the band gap. A posteriori charge corrections are also discussed. A near-identification of an experimentally isolated single spin center as the neutral C[B] point defect was found via comparison of results to recently observed PL and ODMR spectra.