No, if you buy it now you get the entire game whenever it's gets a full release. It's sorta like a playable preorder deal, I guess.
It's a good game, but you'll need a decent PC to play it well and even then right now it's not optimized whatsoever, so even people with high end gaming systems are eating crap when they go to places like Cherno or Elektro...the two biggest towns.
^ That's more of a lag issue than anything else. I for instance run the game smoothly at a near constant 45-50 FPS with 3 year old hardware. The game is (and always was) fairly CPU intensive, so games with similar visual quality will run better than DayZ does simply because they aren't simulating as much as DayZ's engine does.
As for OP's questions, I'd say its completely and totally worth it to buy it now, not just for the price, but also for the fact that it IS a complete game experience. You're not going to lose out on the classic DayZ experience by playing the Alpha, because the Alpha is only lacking in a few features of the mod (mainly vehicles, which many players seldom got to experience on vanilla servers anyway), and has completely overhauled several parts of the mod that were severely lacking.
As long as you don't mind the occasional character wipe (and honestly, unless you're a complete god at this game or are playing it in a really, really boring way, you'll be dying often enough that this won't be that big of a deal) then you can't lose buying DayZ right now.
Lock On: Flaming Cliffs 2. Lets revitalize this game eh? http://www.youtube.com/watch?v=6FUge6ghaIo&hd=1
Don't buy the standalone just yet, it runs pretty bad, get Arma and mod it with Dayz instead. Honestly if you want a zombie game go get L4D or something. Zombies are an afterthought and barely a threat, and everyone just treats this game like a shooter. You're better off getting CoD or Battlefield for more exciting gameplay compared to this