Re: Borrowed Sims 3, still accessible w/o key, but unsure of how to purchase packs

@Animephan100000  You'll need to buy a copy of the base game.  I would suggest waiting until Black Friday week, when Sims 3 content may be 50-60% off.  The EA App sells the "Starter Pack," which includes the base game, Late Night, and High-End Loft Stuff for the same price as one expansion, so $20 U.S. normally and as low as $8 when on sale.  In any event, it's the same price as Late Night alone, and you get a stuff pack thrown in.

Steam requires you to own the base game in Steam in order to buy any packs there.