Are there any reasons why buying one now will screw me over with future expansions?
There might be an expansion with new character cards (they give you special abilities). If those new cards have a new Asmodee design, you could tell which player got one of the new and who got one of the old characters (in the game, they are hidden until a player uses them for the first time).
Apart from that, I don't see any possible real problem of using new cases (which expansion will probably mostly consist of) with the old base game.
Basically, apart from the case cards, you just need a few meeples and some tokens.
Ah, the symbols on the case cards may differ, so they might not exactly fit the look of the corresponding tokens, but I think that would be a minor problem.