Radios:
KO = Teeny tiny receivers, but I've seen people have receivers die or lose bind more than I'd expect. Also expensive.
Futaba = 4pk is kinda heavy. Expensive receivers.
Sanwa = M11x is a great deal right now, because they're trying to clear stock. RX-451R is the super fast receiver, and I've also seen it discounted a lot.
M12 is $$$, but very responsive. I recently got one, and am liking it so far, but have yet to run it on my home track for a proper evaluation. Receivers are decently small, and Sanwa is reasonably priced if you order from Hong Kong. Also, you can pair the M11x or M12 with the Sanwa SRG-BL or SRG-BLS for stupidly low latency.
Spektrum = DX3R is decent. Very light, good for small hands. The SR3100 and SR3520 receivers are really expensive, but they're quite small (only KO is smaller I think). You don't want a lower-end (non DSM2) radio or receiver, as they're more prone to dropouts. I've never had a drop out with the SR3100 or SR3520, though. I'm not sure about their new radio with new protocol, haven't heard anything about it.
Radio Post = I've heard the servos are pretty good, but the couple local team drivers for them dropped them like a bad habit due to drop-out problems. Steer clear.
If you run the front belt toward the battery side, then with the Spektrum SR3100, you can mount it crosswise, and push it inward toward the belt, which lets you move it forward a bit. That will give you maximum ESC room.
With Spektrum SR3520 and Sanwa RX-471 (M12 recvr), you can put them in lengthwise inboard of aft servo mount. Leave like 3mm to the belt, and make sure your servo saver / horn won't hit it or the wires at full lock. Doing it that way gives you pretty ample room for the ESC and a larger spur/pinion.
With the tiny KO receivers, you can probably do whatever you want and still park a BMW 7-Series on the electronics side.
I've never used Futaba or KO stuff personally.
-Mike