Hi there. First of all welcome! I'm sorry I'm not posting this on your thread but I'm very shy when talking about forums and posting.
I want to help you about your TCG questions:
I went to many websites to see if I find the answer to that question(Note: I'm also a total noob). From what I read, the answer is yes. You can mix up cards from different sets to make a custom deck without any fear. Besides it wouldn't be fun if you were locked to pre-made decks.
At the pokemon site you can read the rule changes on the TCG. I assume you can apply those rules to old cards too. Even if there are some text changes or some major changes, just be a ninja and apply them to your old cards. I don't think Nintendo wouldn't be a smart-ass and make you buy a whole new deck just because some rule changes.
This particular question I really can't answer you. I don't live in the same country as yours. Buuuuut from what I know, you can buy a complete deck or some individual boosters. They are usually sold (Here where I live) in comic book stores, some regular shops and, curiously, in some malls haha.
That's it all I guess. Don't rely fully on my information. I'm a complete noob at TCG and I just want to be a friend of yours. Once again I apologize deeply for not posting this in your thread. And if there are any grammar mistakes, please tell me.