V2 is an older version of the T-Rex.
The current versions are the XL (also called "V3" in some places, and SE -- a fully upgraded version of the T-Rex).
You'll probably want either version of the XL, either the HDE (a standard swashplate mix version of the T-Rex), or the CDE (the CCPM swash mix version of the T-Rex). If money is no object, then you'll pop for the SE.
The HDE, standard swash mix, uses one servo to tilt the swashplate fore and aft, one servo to tilt the swashplate left and right, and one servo to move the entire swashplate up and down for collective pitch control.
The CDE, CCPM swash mix, uses three servos, electronically mixed in your transmitter to control the swashplate movement. All three servos work in tandem to tilt the swash fore and aft, left and right, and up and down.
Either version will suit your flying needs, there is not a whole lot of difference in the way the heli flies or feels in the XL version, whether you go HDE or CDE. The CDE version may have a slight advantage over the HDE version since three servos are sharing the load associated with moving the swashplate. The micro servos generally aren't known for having lots of reserve torque or power. If you do your homework and make sure all the plastic links on the balls are not too tight and the controls are easy to move, then the CCPM advantage pretty much becomes a moot point.
If you pick up the CDE version, your TX MUST support a 120 degree CCPM swash mix (and most current radios do). But double check to be sure.
The XL ("V3") version of the T-Rex is MUCH better with regard to quality and control accuracy than the 450X or the 450X-V2. It is also a tad bit longer than the older ones, owing to having the "stretch" tail boom standard. If you are starting out with a T-Rex, start with the XL ("V3"), you'll be miles ahead.
I own a 450X, and a 450X-V2. As time and money permits, I am slowly upgrading both to the XL version by simply replacing the X and X-V2 control parts with the XL counterparts.
If you choose to go with the CDE (CCPM version), you'll need help setting up your radio the first time, as well as setting up the controls.
That help can be found here:
my post, the fourth one down (the LOOOOONNNNGGG one). Read carefully, understand what it says, then follow the steps. You'll save yourself a lot of time and aggravation.
PS -- I don't think people are ignoring your "what does CDE stand for" question, as far as I know, CDE/HDE is just how Align differentiates between the CCPM swash and the standard swash design.
ALSO -- whichever kit you get, do your best to disassemble the swashplate, clean off all the old glue, then reassemble it using CA. All versions of the T-Rex swashplate to date have been known to come apart by themselves, usually in the air.
Refer to Ozydego's post:
or the Align site:
for how to fix your swash. Don't use the bottle of R48 in the kit to fix the swash, this is the glue the factory uses that doesn't hold too well. Use CA carefully so you don't glue the swash solid.