The thing about buying a used disc copy is that the original owner likely registered the code already, so you wouldn't be able to add it to Origin. Sometimes players can manage to get used discs to work if Origin controls the install but the discs themselves are used to install the content. However, it seems kind of hit or miss—some players even get a few discs to work this way, but not others they own.
Origin does have an occasional sale for TS3 content. Expansion packs were 50% off in... May?... and definitely during last year's winter holiday sale. You could also wait for a Steam sale, where packs can be as low as $5 U.S. each. But you'd need to buy a copy of the base game (also $5) as well, so the price would come out even.
As far as whether Generations is worth the money, I'm not sure—I've never actually played with its content. But most people seem to think that it falls somewhere between a stuff pack and a proper EP, and that it's only worth the price if you catch it on sale. The annoying features can be modded out easily enough though, mostly with NRaas Retuner.
