Skyrim's guilds weren't designed to be restrictive against any type of character. Your Orc can get through the Dark Brotherhood just as easily as he could the Companions, Thieves Guild, or College of Winterhold.
well, you CAN do it, but you will have to deal with the guards A LOT if you do not sneak around.
Sometimes you can get bonuses for hiding bodies and whatnot, but I can't recall a mission that required you to not get seen.
Oblivion's Dark Brotherhood actually had a giant 2h wielding Orc as a member "Gogron Gro-Bolmog" . Anyway, yes you can, but be prepared to rack up a bounty on your head. As I recall you get certain bonuses for completing missions in a certain way, but it isn't strictly required.
Most of the quests are just general kill quests. Theres one higher profile mission near the middle of the Dark Brotherhood storyline that will require you to sneak out of a city undetected or else face the penalty of the guards.
Other than that, you should be free to kill in anyway you see fit.
Most of the targets you can just walk up to them, say something to get the swords drawn, and kill them without worry about the guards.
The equipment and weapon rewards you get will be very "stealth-centric", and certain objectives reward you for doing things more articulate or stealthily. It's definitely more geared toward the Bosmer and the Kajiit, but it wouldn't be that much harder as a hack n' slasher.
I like to not use sneak because of my favorite Orc, Gogron gro-Bolmog.
And like Lady renly said, most of the targets can be told you're there to chop their head off. The guards won't give two mudcrabs if you kill them.
You have three option if you go this route.
1. Walk up to target and smash him or her with your weapon and run away.
2. Walk up to target and smash him or her with your weapon and pay your fine/jail time.
3. Walk up to target and smash him or her with your weapon and kill everybody that attacks you.
A conspicuous assassin doesn't sound like that bad of an idea.
Nope, even the npcs have some obvious kill people, like that destruction magic guy. I sure he wont be sneaking around, he'd just fling fireballs at the target and be on his merry way.
Oblivion's DB had more quests that revolved around stealth, but no, in Skyrim you can just waltz in and cleaver your target apart. Forget espionage.