I would like to start playing, i know it's a bit late. But what should I do?

1- If I prefer to play offline, what / version should I buy/download?
Any websites to download for free?

2- If online, what should I purchase/download?

Means anyhow i need to play online??
I dont mind buying. but should i buy the main version, finish then buy the expansion?
is the expansion extra (move levels/quest) than the main version?

So if i buy both, with expansion. izit better to buy a hard copy/
Or just but online where they will only sell you the key, and you download from somewhere else?

Another one, if I buy from someone else, izit safe?
meaning like if there is only 1 key, only 1 person can online at a time, meaning if he still uses the same key, then i cannot play?

Thanks guys...
I will try and update later