I also recommend all three expansions. I've been trying one of the mods (Commander's Edition). It's pretty good, too, and nerfs the Marauder head-hunting bonus.
I agree that Marauder + 9 tactics skill in the base game gets pretty ridiculous.
Overall, though, between the added flashpoints, maps, and Mechs of the three expansions, the game is almost becomes very re-playable in career mode.