I know barely anything about this game (bought it on a whim when it was 90% off in the Big Winter Sale), but from what I remember reading somewhere in this forum, the "complete" version is actually the older version of the game, while the current officially supported version incorporated a lot of mods and new features. But I only tried the newer version, so I can't say firsthand what all the differences are.

The newer version has no multiplayer mode. What's up with that?

I guess this has already been asked and answered (and the topic was just seven posts below this one in the Postal series forum): https://www.gog.com/forum/postal_series/no_more_multiplayer_in_postal_2

Because RWS didn't upgrade the game since 2014. Just input master.333networks.com in DirectIP and you'll get the warning that your version is too old.