Welcome Doug! I agree with Gregg; from what I've read the Tekin is a great choice. I chose the Castle Creations Mamba Monster, as I've had great luck with CC products in the past (from a RC10B4). I also prefer the bullet connectors on the MMM over the soldered posts on the Tekin.
If you're looking for a battery that's less expensive, I'd highly recommend Zippy or Turnigy from Hobby King. Inexpensive and well made. Probably not as good at Reedy, Thunder Power, et al, but should be just fine for bashing or getting started in racing, IMO. I just got my car assembled and will be using 5S (18.5V) and 6S (22.2V) Zippy LiPos that are 40C rated. Another good inexpensive source for batteries are ProTek from A Main Hobbies. That's what I use in my son's 1/16 Revo, and they've been great batteries.
I wouldn't go lower than 4S (14.8V) on the battery. I think you'd be disappointed with the lack of speed (especially with an 1800kV motor).
Also, watch the RC8 build tips on YouTube. Great stuff, and sand the diff cups. Makes a huge difference, IMO. I was surprised at how "unflat" the diff cup flange was/is. I would also suggest using slightly longer screws. IIRC, the ones I used were 2mm longer and felt quite a bit more secure.
For the updated diff o-rings look for PN 89121. The AE web site has a picture of the updated o-ring kit on the RC8Be parts page. There are 6 small black o-rings and 6 large clear o-rings. Updated o-rings are clear, older ones are black.
Gregg, my problem with the reciever is that there doesn't seem to be a good spot for it in the receiver box. The way I have it mounted right now, the wires coming out of the servo and ESC connectors have to make an immediate 90 degree turn to clear the cover. However it dawned on me on the way to work this morning that the receiver is small enough that I might be able to turn it on its side and mount it that way. That would solve my wiring issue. As for the antenna, I wouldn't want to coil it up inside the box, but I might make a laydown antenna inside the body.
Anyone ever shorten servo wires before?
Also, any idea where I can source the bullet connectors used on the CC motors for the MMM ESC?