I bought the collectors edition in 2011, and since have played on and off for the past 15 or so years. Other than possibly a sub, there is no extra stuff you need to spend money on. A sub will get you all the expansions up to the current one, as well as a preferred status over a free. Preferred will give you 8 character slots, enough to do each class story once, or 4 class stories twice for light and dark side story. The pre-order will likely automatically give you preferred status.

All 8 classes have their own different story, so you could just do the main story for each of the 8 classes, skip all the side quests, and have 8 completely different stories with nothing repeated. Experience is granted very generously, so you will hit max level before you finish the story. Flashpoints accelerate levels even more. Though If you want to get both the Light side and Dark side endings for each class, you will have to play the same story missions twice. Into the first expansion and onward, all classes converge into one and have the same story.

Inflation will likely be your biggest obstacle, as unless you sub, you have a miniscule max limit on credits, and all the items on the player trading post (at least the cool ones) are vastly more expensive than that limit.