Hi all and apologies in advance for asking dumb questions but I am rather new to this.
Initial research showed that getting a pre-owned Phantom 2 Vision + is a good compromise between quality and cost to get some hands on experience before spending painful amounts of money.
The challenge I found is that it is far from clear which version 2 is which. Some say the different versions are: Phantom = mk1, Vision = mk2, Vision Plus = mk3.
Other sources state that the best model in the Phantom 2 series is the Vision Plus V3.0 and that there is also a Vision Plus V1 and V2. I have looked at the DJI website but could not find the answer there.
Who can tell a bright green newbie what the definitice answer is here?

Many thanks in advance

The Phantom 2 Vision was released in the fall of 2013.

The Phantom 2 Vision Plus was released in the spring of 2014.  There's a sticker on the bottom of mine with a bar code and some numbers followed by "V2.0"

I believe this was in reference to the fact that the P2V Plus was the second version of the Phantom 2 Vision series.

In the late fall of 2014, another Phantom 2 Vision Plus became available, but was called Phantom 2 Vision Plus V3.  It included quite a few improvements over the original Phantom 2 Vision Plus including composite threads on the props, different prop angles (9450 vice 9443), stronger motors, a wheel on the controller to move the gimbal up and down, etc.  

I assume the V3 is meant to mean it was the third version of the Phantom 2 Vision series.  

I don't believe there is a such thing as a Phantom 2 Vision Plus V1.  Also, nobody ever called the Phantom 2 Vision Plus a Phantom 2 Vision Plus V2.  They just called it a Phantom 2 Vision Plus.  When the Phantom 2 Vision Plus V3 was released, the "V3" did confuse some people, and it still does.  

Hope this helps.  

Hello Mark and thanks for your fast reply.
What you mentioned seems to confirm what I found from another source so that is encouraging.
The bad news is your and the other info mentioning the ongoing confusion for many including myself.
Am I correct in thinking that there is a noticeable advantage that the V3/Vision Plus (strike through where not appropriate) over the V2?
If so, what is that advantage in practical terms and how do I tell whether the kit I'm looking to get is the best/latest Phantom 2?
Also, as I'm looking to buy this used, is there a 'reasoable price' for it?

Noticeable advantage?

I would say the V3 AC has a number of advantages - otherwise DJI wouldn't have bothered with them.  Many of them are noticeable.  But even the ones which are not noticeable are noteworthy and worthwhile.

Re the AC, I have the V2 with the weaker 2212 motors, but I am perfectly happy with these motors.  However, as you can see in the video, the 2312s are stronger and have better bearings.  Also, I have the V2 compass.  The V3 compass is supposedly better and is encased in plastic, but I've never had a problem with my compass.  I also have the non-perforated wifi module.  The V3 has a perforated cover over the wifi module to aid in cooling.  I would prefer the perforated cover because there have been many complaints of the wifi module over heating and burning out, but so far, my module has not overheated.  I have the 9450 props which are V3.  It's easy to buy 9450 props and put them on a V2.  I have the V3 ESCs (they are actually called V2.1 ESCs as written on the board).  IIRC, the advantage is they will work with both motors, whereas the older ESC (v1.x) will only work with the weaker motor.  The V3 has a slightly bigger GPS shield stuck to the inside of the top shell which extends a little further up the arms.

I think the biggest advantage to the V3 is the controller.  I would not use the old controller.  Even if I bought a V2, I'd get the better controller for it.  Get a NPVT581 which is the V3 version.  I think they're around 50 USD used.  The transmitter is stronger, it has a lithium battery, and it has the gimbal wheel control.  

In summary, I have

original Phantom 2 Vision Plus V2 AC with new 9450 props and new ESCs V2.  

and V3 controller.  

Watch the video.

Don't confuse a straight Phantom 2 with the Phantom 2 Vision series.  They are completely different.  

Hope this helps.