Tacmarines...kinda suck, imo. Jacks of all trades, masters of none, which has never been my interest point in a game but to each his own. They can plug holes pretty well. They're OK but not necessary in a game the way they are in the fluff, ESPECIALLY not after everyone can buy the Forging the Bond Talent and benefit from other Chapters' Squad Modes (albeit at higher cost, but Cohesion is laughably easy to stockpile to absurd levels under the RAW).
Devastators are a little different, I consider them pretty important especially as they are the most effective Specialty to deal with Hordes. It will probably be easier to overwhelm a party lacking a Devastator with hordes than without, and hordes above Magnitude 20 get exponentially deadlier with every +10 Magnitude, such that they need an effective PC counter. That being said, psykers are an excellent horde counter, and anyone can bring along a heavy flamer or heavy bolter. And properly-built assault marines can chew through hordes too, but they'll be at higher risk since horde melee attacks cannot be dodged/parried (unless you're a Tyrannic War Veteran).
Really you have your most important specialties: Apothecary (to keep everyone alive, and even decent at in-combat healing too) and Techmarine (for all the special technical stuff that most Astartes can't learn early on). Both of them are still efficient deathdealers too!
As to your mortal character: DW and Ascension do not play well together, in part because of mortal squishiness, in part because a mortal skillmoney will remove any skill-related challenge that might actually frustrate an Astarte, in part because Ascension is not a well-designed game addition. And part of this is speaking from experience of having a Desperado in my Deathwatch game for awhile, not just general Ascension-hate. Also remember this guy cannot benefit from Squad Modes AT ALL, which will decrease his survivability even more.
Edited by Kshatriya, 19 October 2013 - 10:27 PM.