Hopefully someone will correct me if this is at all off-base. I'm not a native Vray user although I do own the software and am getting into it. I'm primarily a Maxwell render guy but my understanding is that they essentially utilize resources in the same way. With that said,
RAM doesn't increase render speed. That's an important and misunderstood idea. It seems like it should, and oftentimes I.T. people will tell you it does, but when it comes to rendering it just doesn't. The only benefit to RAM in rendering is that it determines the size of scene you can render. Let me clarify. You've got a normal size exterior scene. It will take the same time to render with 8gigs of RAM as it will with 64 gigs of RAM. Let's take another scene, a full masterplan development, tons of polygons, lots of trees, etc. The machine with 8 gb of RAM won't run it SLOWER, it just won't run it - it will crash. The machine with 64 gb on the other hand will be able to render even a huge scene. Besides polygon count, it also determines the resolution you can render. You might be able to run a huge scene at 1000x600 or a small scene at 6000x4000 if you don't have enough ram.
So to answer the question of how much RAM do you need, you'd have to first ask, what's the largest scene I'll ever render? If all of your scenes are being run from sketchup then sketchup will almost certainly reach a poly limit before Vray runs into a RAM issue. I wouldn't be surprised if you could get away with 4gb of RAM since you're only going to be able to use low poly trees and mid poly cars without sketchup being horrible to work with. If you use 3ds max or another program that deals much better with high poly scenes then you can easily run out of RAM at render time.
I would suggest that 8gb would be a safe minimum. I've used more than that on my 18gb machine but usually that's because I'm running other programs at the same time on my workstation or because I'm not limited to only Sketchup geometry.
So then, the speedof the rendering comes completely from the CPU. Get the fastest CPU with the most cores you can afford.
Maxwell (and presumably Vray) is pretty linear in this regard. In other words if you're looking at a 4 core processor at 3.4ghz vs. a 6 core processor at 2.9ghz you can pretty much tally up your total ghz for each cpu. The first cpu has 13.6ghz total (4 x 3.4 = 13.6) and the second has 17.4ghz (6 x 2.9 = 17.4) so you can bet that the second processor will be faster at rendering (and you can bet that it'll be faster by about 25%). So of the 3 options you gave, the 1st options is head and shoulders the best, although perhaps this information will lead you to look to other options.
Couple notes.
*If you were talking about workstations it makes sense to consider getting a cpu with a high clock speed since modeling only uses a single core. Since you're dealing with a farm, that doesn't apply to you though.
*Hyper threading throws a bit of a tweak into the cpu calculations. I've seen between a 10%-20% boost in performance above what I anticipated because of HT
*Rendering is one of the few functions which will tap out whatever cpu power is available. It will run every core of the cpu at 100% for however many hours your rendering goes for. That said, be cautious about overclocking as it could reduce the life of the cpu and perhaps cause instability, both of which can cost time and money.
-Brodie
