There is only one official version. One price.
In the options in the game you can change a bunch of stuff to turn monsters off, make it Peaceful, Easy, Normal or Hard. You can even pick how you want your world to look, as in flat, random, mountainous, etc.
You can set it to Survival mode to play it normally and mine blocks, build and defend yourself from hostile mobs or you can play in Creative mode and have all the items the game makes in your inventory and not worry about the hostile creatures hurting you, they go neutral in creatve mode.
There are hundreds of mods made by the community that you can add to make it play however you want.
There is a Minecraft WiKi that will explain many many things and to answer any questions you may have. Go to minecraft.net which is the games website and it will have links to forums and many other places to go.
If your worried it may not play well on a lower end or older computer then it does have video settings now that you can change to make it play better.
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It costs £4.99 but there is also a lite version, which is available for free. However, the lite version has fewer features.
Yes
You can play the demo version or Minecraft Classic on the Minecraft website.
Yes, you can play minecraft in a ipad (if that's what you meant). Download minecraft lite for free.
Currently, Minecraft's newest version for PC is 1.4.7