If you are new to this, starting with an 840 is ambitious, but with what you have now, you want to target props of 13 to 15 inches, and get pretty low kv motors to drive them. The first complication I can see is you may need a battery with a higher current rating than 30 to drive four motors which may max out at more than 50amps. And many of the motors you look at will call for 4 to 7S batteries!
Here's one to look at: (I chose because it includes S, current and thrust data)
http://www.hobbyking.com/hobbyking/...Outrunner_550kv_46_Glow_.html?strSearch=550kv
With a frame that size, you may also wish to consider an OctoCopter, mounting four motors half way back on each arm. In any event, you want the ESCs to be rated around 10% or more above the max current of the motors connected to them.
If your 3S lipo doesn't work in the flight mix, don't worry -- you'll be able to use it to power your FPV gear on the ground. Don't make the 3S you have a criteria for choosing anything (that may not work in the end).
I assume you're building large to get payload, flight time, range or all three. If you want long range, don't forget to consider LRS options before buying anything that smacks of a radio. Not all flight radios adapt to all modules or frequencies (or telemetry) Lower frequencies typically carry farther, so don't get sucked into a 2.4GHz radio with 5.8 FPV if that's not going to do what you want. And if long range is in the picture, you'll want the center deck (at least one of them) to be pretty large, so you can get some separation between your various transmitting and receiving antennas (FPV, telemetry, GPS, and flight receiver). The closer they are, the more they tend to stomp on each other.
Did you fabricate that 'X' frame? If so, you should be able to fabricate motor mount plates, and if you have heliarc gear, you might as well attach them that way. The center "deck" can be fiberglass or carbon fiber, probably two or more plates, spaced to hold your controller, receiver, ESCs, battery, sling a camera, and mount your GPS in a dome on top, so the GPS is above everything else (looking horizontally across the craft), especially the carbon fiber props.
Maybe it would be good for you to tell us what you intend for this (quad?) to DO. You had to have something in mind to make it this large, and apparently not collapsible! At least I see no hinges or pivots anywhere. And by the way, how do you intend to transport this beast? If you built that frame, I'd think about engineering some pivots! Otherwise, you'll need a cattle truck to haul it.
