How do you get the Silverback unlocked in Horde? ie What has to level up, and what's the quickest way to get to it?

IIRC you have to level Turrets up to 8 or so, for sentry you have to level the decoy

You unlock it by spending a total of 170,000 dollars on barrier fortifications in Horde.  
 
Unfortunately, there isn't a quick way to do it. Be sure to remember that you aren't restricted to only a single command post during a game of Horde, and that having purchased multiple command posts opens up many more barriers for you to build, rebuild, and repair. Sometimes, allowing a barrier to be destroyed so that you may rebuild it entirely is the best avenue for your cash expenditures, while othertimes it is more efficient for you to upgrade the barriers (from cow-traps to razorwire to electrowire, so on and so forth), since the cost for repairing a high-level barrier is typically more expensive than the cost of building a minor one.  
  
Just chip at it. Eventually it will come along. Be mindful that a lone Silverback costs 12,000, so you may find yourself saving for a while before you can purchase it. Though, don't be afraid to ask for cash from your fellow Horders, since they can pool money with you before the round begins. 

For Silverback I believe you have to spend about 170,000 or so on barricades. If you access your stats on the game menu and look at the horde mode details it lists what is required for them to be unlocked as well as exactly how much you have invested so far. Quickest way to unlock it would be to just exclusively focus on building barricades and repairing them. It will take quite some time though.

Cow-traps? Really?

http://en.wikipedia.org/wiki/Caltrop

Gruff voices arnt the best for pronunciation. I thought it was cowtrop so ya.