Awesome, thanks for the reply and advice! This is great news to hear. I also imagine that the expansions will help to alleviate some of the competition for certain strong cards, right? I intend in buying a few saddlebags and a pinebox every few months until i catch up.
You're right, the expansions help by offering interesting new cards, encouraging people to switch from certain base set cards and encouraging whole new decks using completely different values. For example, buying "Faith & Fear" and getting access to the "Faster on the Draw" action card might encourage a shootout deck to try playing a deck with lots of 6s, freeing up lots of cards on another value.
Doomtown's deckbuilding rules help reduce the competition for things that might be "staples" in another game, as you might not include them in your deck if they're on the wrong value (not part of your shooting structure, too low for your spell pulls etc).
Good strategy on gradually expanding the card pool - according to a recent Facebook post the upcoming Kickstarter should have an option to pick up some of the old expansions, so that might be a good way to get hold of them.