I've seen some other conversions that looked pretty good. You could still see that they were from standard marines, but no one had a problem identifying them as daemonhunters. Mostly, you just need to get the storm bolter and the halberds right. The rest is up to taste, really. Twin bolt pistols can make a great wrist mounted storm bolter, and the halberds are pretty easy to convert from chainsword power generators, rod, and plasticard for the blade. Also, any kind of 'knight' styled helmet, like those found in some marine/chaos marine plastics and especially in WHFB knight sets can really make a visual impact on the model. Otherwise, just using lots of purity seals and inquisitorial icons would work.