First things first: Don't worry about a Deathwatch Kill Team Formation, as you have to adhere to the Kill Team Detachment for which a Kill Team Formation does not, alas, qualify.
For Deathwatch, this pretty much means 'grab 6 Power Armour models and gear up to taste'. Deathwatch is expensive pointwise to start with and 6 is prolly the max you can get and still get some gun upgrades. A biker for the sixth is not a bad choice - faster and tougher makes him a good harassment choice.
This means fully half your team will be Specialists, plus a fourth will be Leader. Pick them wisely.
Unless you're looking at an 'indoor' mission with no open field, Stalkers aren't a bad choice. Table shouldn't be more than 4x4, your shortest range on a Stalker is 24" and everything else (so every shot that doesn't absolutely need to go through 3+) is longer, so find your Stalker-armed marines a sniper perch somewhere mid-forward in your deployment zone and just shoot from there.
You need at least one Frag Cannon I think, for anti-vehicle duty. Getting a Meltagun in range gets iffy... Vehicles with 33 or less total AV are not just Rhinos, you might meet such niceties as a Razorback(11/11/10), Immolator (11/11/10 again, particular favorite of mine), Chimera (12/10/10 with a multilaser and heavy bolter as standard issue), a squadron of Armoured Sentinels (12/10/10, comes with a Multilaser and can pick up a Missile Launcher that'll go straight through your armour save for 5 points more)... and that's just the basic Imperium vehicles.
Good news, Mission Tactics will be invaluable because the enemy's only going to have one (MAYBE two) battlefield roles in play, so you won't be missing much. Not much to worry about Gets Hot when firing Vengeance rounds.
Not so good news, you're going to be outnumbered, as per usual. 6 models (you can lose 4 before taking break tests), 6 wounds total... which isn't very many at all given that some of the other armies can easily put four times that on the table. Worse, if you're not outnumbered, you're probably looking at something like 6 Wraithguard, as nobody told the Eldar they could not bring D weapons (You could take those, though...).
Ask not the Eldar a question, for they will give you three answers; all of which are true and horrifying to know.
Last edited by Shandathe; 03-10-17 at 12:15 AM.