I think it should be pointed out that 6th Edition allows Allied Detachments for
every army, so you could easily field a
WH/
DH-themed army with
IG Allies using current codices. So, if this is what you are after, then the current rules would have you covered as well.
Still, if your local group would let you field older codices, that is of course an option, too. Both the Witch Hunter and the Daemon Hunter codices are currently available as free downloads from
GW's website (minus a chunk of fluff sadly), so the books themselves would not even have to be purchased.
http://www.games-workshop.com/MEDIA_CustomProductCatalog/m1050287a_Codex__Daemonhunters
http://www.games-workshop.com/MEDIA_CustomProductCatalog/m1050291a_Codex__Witch_Hunters
Since you mentioned that fluff is the most important thing in your eyes, I'd say that the choice of your Inquisitor's Ordo could depend on the style you want to see in your army. The Witch Hunters of the Ordo Hereticus tend to be very militant, fielding large contingents of inducted Imperial Guard or Planetary Defence Forces, although the Inquisitors themselves tend to wear "comfortable" clothes such as large coats and silly hats (medieval style). In contrast, the Daemon Hunters tend to prefer smaller forces for a more secretive approach, but the Inquisitors of the Ordo Malleus often wear heavy armour to survive an encounter with their daemonic foes. The Alien Hunters of the Ordo Xenos fall somewhere in-between. It should be noted that all these things can be done with the current
GK Codex and the Henchmen rules, as already noted by Melcavuk.
If you're interested in the background of the Inquisition, I also recommend checking out the following Games Workshop resources:
Inquisitor RPG rule- and sourcebooks
Inquisitor characters
Melcavuk wrote:Grey knights are probably the most cost effective route since all sisters of battle are still old metal molds and have no rumours of a line refresh anytime soon, and offer an amazing variety of models for you to use.
Although personally I don't give much on rumours, it would be wrong to say they have none - check the various
SoB threads. Also, whilst the miniatures may be old, they look quite beautiful and the metal feels nice in one's hands. Lastly, due to
GW's price increases across all ranges, the cost difference is not as large anymore as it used to be. Miniature for miniature,
GKs are still more expensive - you just won't need quite as many
GKs as you'd need Sisters for a proper army.
There are a number of arguments against starting an
SoB army now (chiefly Codex availability and contents), but I do not believe the miniatures qualify as one.