Hey there!
For starters, you need to be aware that there are now two different versions of the Deathwatch around. The small elite Ordo Xenos kill-teams from
GW's vision - and the completely independent
FFG RPG version that may occasionally show up with tanks, dreadnoughts and battle barges.
Personally, I stick with
GW's version, but I thought you should know that there exists a different interpretation. Which may be more up your angle if you want to make it a full army. It comes down to how close you want to remain to what Games Workshop writes about the setting.
As a "workaround", you could perhaps also designate one of your Marine squads to be a Deathwatch kill-team and paint the rest as a normal Chapter, kind of as if they're allied forces when they are in fact a single army ruleswise, y'know?
Alternatively, you could use 6E Ally rules and split your army into two components - the Marines, and a small Ordo Xenos "add-on" using
GK rules with 1 Inquisitor and 1-2 squads of Deathwatch-coloured Marine squads and 1 squad of Henchmen as ISTs.
Or go the
FFG route and make the entire army Deathwatch and just accept that this isn't how
GW described them.
Also, Lexicanum is (a) not up-to-date on the Deathwatch and (b) would have to decide which version of the Deathwatch they want to feature in the first place. This wiki still likes to treat the setting as if there was a single "true" representation of it when this simply isn't the case. There's "thousands of different, overlapping interpretations in the minds of writers and gamers", as Gav Thorpe once explained it - or numerous "different lenses through which you perceive the setting", if you'd prefer ADB's wording. You can't just do a wiki and throw everything from any
40k source ever into it and expect the end result to be consistent ... but that's just my opinion, I guess.
As for
GW fluff sources, there are some you may be interested in if you'd favour their view on things:
Here,
here and
here are a couple PDFs hosted on
GW's website for the Inquisitor
RPG. Just search for the keyword "Deathwatch".
Here is an excerpt of the Index Astartes II dealing with the Deathwatch in detail.
White Dwarf #306 also has Deathwatch Kill-Team rules, but they are outdated by now and it doesn't contain
much fluff in addition to the above, other than the
DW preferring to recruit from Codex Chapters and that they get their equipment and training from the Inquisition, and that their characteristic Stalker bolters are basically just standard boltguns equipped with a targeter and silencer, and loaded with Stalker silenced shells.
Also, the
DW has been talked about a bit in
this thread a couple weeks ... months ago.
Hope that helps! o7