storm bolter/combi bolter: i would make it simple and keep them the same. SH rules are 40k rules-light and they are similar enough to just translate them into the SH rules as they are
melta/flamer combi bolters: meltas get one shot that kills a single target on 3+ on 2 dice (i would say 2+ on one die but it should have a good chance of taking out a brood lord too). flamers get one shot like a heavy flamer, just need 4+ to kill stuff. the bolter part only gets 1 dice , can never jam though.
reaper auto cannon: 2dice killing on 5+, sustained fire, no chance to blow up. other then that, same as AC
power fist/power weapon: one is faster, the other one more powerful, i suggest keeping them the same.
sorcerer powers: i would make one warp time, costs 1 to cast and the sorcerer gets to re-roll all hits (ranged and CC) till his next turn.
another one could be aura of terror (or tendrils of the warp or whatever), 2 points to cast, cast on a section with persistent effect, all movement costs are doubled till the beginning of his next turn.