For the shafts, you can probably save a few bucks and use Revo sliders. They are plenty strong and cheaper to fix/replace if something does go wrong.
Upgraded diff cups are definitely a good idea. If you want to go all out, I believe there are better ones using 4 spider gears instead of 2.
After you see the power of BL, you'll probably never use first gear again. Removing first gear, linkages, shift fork, shifting servo, etc saves some weight, rotating mass, and gives you a little extra room (because of the missing servo). If you do that, all you need is one idler gear. And you might want to pick up just the locking mechanism of the single speed conversion - it makes the tranny less sloppy and is easier on the gears since you don't have the slop from the dog gear.
For batteries, those look good. If you want to save weight, you could go with a couple of lipo packs. You can get packs that have much more runtime for the same weight (or less). Of course, you'll need a lipo charger and I recommend a balancer.
The Quark is a solid controller - I like it a lot despite the fact it won't work with 6s lipos very well. But you'll be using 14 cells, so you're perfectly fine. You might want to pick up the
heatsink, or make some type of additional heatsink as the Quark does tend to run warm. Depending on when you plan to purchase all this, you may want to wait for the Mamba Monster Max (if it ever comes out). It is supposed to have some pretty good specs and might be cheaper.
The Neu is a very good motor. The 2.5d might be a little too fast with a kv of 2600. Depending on gearing, I'd get the 2d instead. Up to you. You can approximate the speed of your setup by using the
top speed estimator calculator. No sense in getting a faster motor than you need since all it will do is decrease runtime and pull higher currents than necessary.
The Strobe slipper is a good idea as you'll melt the slipper pegs in the stock setup if you have any slip set. And yes, you'll need that Ofna 51T spur. It's a Mod1 pitch, so you'll need some Mod1 pinions, which are available here as well.
You should also pick up a
UBEC, which converts your main battery voltage (16.8v in your case) to 6v for your receiver and servo(s). And it's a switching regulator, so it'll stay cool. That way, you don't need a receiver pack. 14 cells is way too much to use the ESC's built-in BEC.
I've never had an actual emaxx, but I have used some emaxx parts in my first Revo conversion. So, other people may have additional suggestions...