I heard about that edition too but there's no sign of it on Steam. Arma II's DLC was a bit lame, though, they'd add all the weapons, vehicles, characters and locations to the base game for free (for compatibility when playing online), which was awesome, but the ÂŁ8 you'd pay for the DLC itself got you high resolution skins (wow) and a singleplayer campaign which was always a bit lacklustre. Operation Arrowhead was the only thing that was really worth getting, that was a full expansion which was optionally stand alone and it made huge improvements to the engine; I'm guessing if there is a large expansion for Arma 3 it won't be covered by the complete edition and the smaller DLCs may not be all that worthwhile anyway, assuming they still add all the assets for free (which they may not).
I was just typing up a response to your second part under the assumption that Arma II and Operation Arrowhead (which is a must have, maybe even more than Arma II itself!) were still really expensive, but at ÂŁ3.99 and ÂŁ5.99 respectively I say do it. It's a very, very unforgiving game (because it's really more of a simulator) but it's awesome at the same time. The Day Z mod (which now has Steam integration for ease of use) adds a great deal of value to it too.
Edit: Oh, this is just a one week deal, 50% off until the 12th. You can get Arma II, Operation Arrowhead, British Armed Forces, PMC and Army of the Czech Republic for ÂŁ12.49, which I'd definitely really recommend. I mean yeah, I just said the DLCs are kind of worthless as most of the content is included, but ÂŁ2.49 for the high definition textures and extra missions is a good deal.
Edit again: I've just played the alpha for a few hours. Not a lot of content (shocking, right?) in terms of missions but I haven't touched multiplayer yet, the graphics are stunning but it's quite a heavy game to run, with my 680 on one monitor and reasonably high but not completely maxed settings I was getting an average of 40fps on foot, 25 in a vehicle (because the picture in picture feature is quite power hungry, but you can't turn it off because you'll lose the ability to use vehicle weapons) and 20 in the helicopter (I guess because of having to draw 2km in all directions?). The default settings, which were everything maxed, on triples I was getting 20fps on foot and 10 in the chopper, didn't try the vehicle showcase.
I think this might be the game that motivates me to get another 680!