This question gets a bit tricky to answer... Starting from scratch, if you buy just vanilla WoW, you can experience the original world as it has been destroyed by the Cataclysm expansion. You can level up to level 60, but cannot access professions, areas, races, or other such things from any of the other expansions.
In order to access Outland, jewelcrafting profession, and Draenei? / Blood Elf races and starting areas, and level 61-70, you will need vanilla WoW + the Burning Crusade expansion.
In order to access Northrend, Inscription profession, and Death Knight class, and level 71-80, you will need vanilla Wow, Burning Crusade expansion, and Wrath of the Lich King expansion. Oh, and a prior level 55+ character on your account to play as a Death Knight.
In order to access the misc Cataclysm level 80+ areas, Archeology secondary profession, Worgen / Goblin races, and level 81-85, you will need vanilla WoW, Burning Crusade expanion, Wrath of the Lich King expansion, and the Cataclysm expansion.
Those are the restrictions. You can play on whatever realm you want with whoever you wish. You just may not be able to reach all the areas they can.
You can trial every bit of it with the 10 day trial, though I don't quite know how they treat it when your time is up.
This is outdated now; please see Ben's answer.
The World of Warcraft client is a 32-bit program. As such, running it in a 64-bit OS provides no benefit except possibly extra available memory. In fact it may be slower on a 64-bit OS; Windows 7's WOW layer, which essentially translates between 32 and 64 bits, is naturally slower than native 32-bit Windows 7.
As for the memory, a 32-bit OS generally only allows for 3-4 GB (see info for Windows). This should be enough for WoW and the rest of the system, depending on what you have running in the background. If you want a lot of other apps using memory, then you should probably go 64-bit with more than 4 GB RAM.
Again, I can't see this being a problem. The hardware requirements for Wow: Cataclysm state that only 2 GB of total memory is required, so having 4 GB in the system should be plenty.
That all said, if you're building a new computer this shouldn't be an issue at all. WoW should run great if you have a good enough graphics card to support the resolution and maxed options. In that case I would go with 64-bit myself, since there will be more and more software that takes advantage of it (and you might want to play crazier games that need more memory than WoW does).
Best Answer
The short answer is no. The game has a level cap of 100 (assuming you have all expansions), so you'd get 20% of the total leveling experience and none of the post-game (what you do when you hit level cap) content.
The starter pack is roughly analogous to a demo in a singleplayer game in that regard; it gives you enough of the game to understand whether or not it's something you'd want to spend money on.