I have made the mk5 armour from a standard mk7 set, using the tutorial found here.
The head is from the marine bike sprue, and the bionic arm from the FW veteran squad (got a few bionic left arms lying around from those packs!) The chain weapon, used to cut open marines armour (and to treat them, or maybe get to the progenoid glands?!) I think is from the old 2nd edition ork CC weapon sprue.
Sadly I don't have a substantial bits-box while I am away from home. I'm hoping not too many people will identify his medical kit as a Tau accessory (did marines get tried for Heresy back in the Great Crusade? Have some Tau travelled back in time so the marine could get hold of this piece of kit? There are all sorts of Terminator-esque time-travel paradoxes involved here :) )
Hopefully when painted, it will look a bit less obvious, and I may well modify it slightly.
The scanning array on the backpack is from the vehicle accessory sprue. I didn't have any access to cabling for the torso, so I resorted to using greenstuff. I actually think it has come out a bit better than the cable I have used previously, and will have to see how it looks when it is painted.
As you can see he still needs basing, and some mold lines cleaning, I only seem to notice them when I look at the photo :)
As for colour scheme, the guys on the Great Crusade website have pointed me in the right direction, and there is evidence of Apothecaries in both the '40k' colour scheme (white armour) and also in standard Legion colours. I think I might go with the latter route, as it ties in more with how the Librarians and Techmarines are represented, and also paints the marine as a specialist who is still part of the Legion. He hasn't gone off to train in some ancient special apothecaries sanctum, it's been handled in-house as part of the Legions operations. Although I realise that his armour will be white in any case, so it makes very little difference! :)