Yes, that is a typical phrase when we close a letter. As a direct translation to English, that means: ”Please take care of yourself and I hope you’ll always be fine.” Even though it’s a common phrase in letters, you can still use daily conversations with anyone. However, about your third wondering, いつも気をつけてね isn’t the same meaning and we seldom say いつも気をつけてね. I guess you might regard　気をつける as “take care”, but when we use 気をつける with no objects such as 体 or 健康, 気をつける almost always means “be careful”.　I’m sure this is a very difficult aspect of Japanese language, but please don’t worry because the listeners will surely understand and appreciate your kindness even if you say so.

That's 「前もって」. 前 means front or forward in Japanese. So "Thanks in advance." is 「前もってお礼を言います。」,''I'll pay in advance.'' is 「前もって支払います。」,- ''I prefer to plan things in advance.'' is 「前もって計画しておきたいです。」　I wish this would be a little help for your question.

I'm sorry to forget answering the other question of yours. 口コミ is reputation or rumor spreading from people to people. Probably it comes from 口 and コミュニケーション（mouth and communicaton）. So you can use it like this,「彼の悪いうわさが口コミで広がる。」 (His bad reputation spreads from people to people.)

Unfortunately, both of these 擬音語 you suggested are not appropriate in stead of "Knock, knock." Thas't because どんどんsounds like a huge sound, so it sounds like some strong and wild man is knocking on your door so hard!!：) And also, ぽんぽん sounds like a bit comical or childlike, so it sounds like some kind and pretty creature is knocking on your door with something soft. Probably what 擬音語 you want is こんこん. This is often used for "Knock knock" and you can find it in Japanese picture books.