What do I need to buy to get the full experience of Total Annihilation?
TA is awesome just by itself! The game is (was) totally revolutionary at the time and the ability to create endless units is one of the funniest aspects of the game.
The expansions are not necessary, but it's nice to have them when you want to see new units, etc.
Buy the full game with the expansion packs. Install it without them, play for a while and then install them.
If you want to play with mods, install a better AI or install community-released bug-fix patches, you better get it with the Core Contingency expansion, as most mods and game patches require it or will work much better with it. Battle Tactics is more or less optional as it is mostly just new campaign and skirmish maps, anyway.
And if you're gonna buy it, better buy the GOG.com version as it is setup to be fully-patched (CD check removed and modified to play music without CD, among other fixes) and readily used on Modern OSes.
Get the base game and The Core Contingency and you are good to go. Check out http://www.tauniverse.com/play-ta/ for info on how to play online.
Where to play online
For multiplayer, all you need is TA itself (as well as the IP address of the person / party who is hosting the game), but you can organize games over instant messaging programs, like AOL Instant Messenger (AIM), ICQ, or Windows Live Messenger (MSN). However, if you'd rather play or organize games with the help of a specific gaming client or site, check out
GameRanger
PhoeniX WorX
WarZone
Required downloads
TA 3.1c patch: Upgrades TA to the latest version (3.1c).
500 unit limit patch: Changes the maximum in-game unit limit per side to 500.
Extra Cavedog units: The 6 post-3.1c patch official Cavedog units packed into 1 file.
Extra Cavedog maps: All the extra maps Cavedog released after the expansions packed into 1 file.
TA Demo Recorder (99b2): The latest version of the TA Demo Recorder, adding several functions to TA, like shared LOS for allies, cheat detection, and game recording so you can watch replays again later.
Well I suppose it depends on your definition of "the full experience".
If you're looking to get a bigger range of units and maps, then I highly recommend the expansion packs. Although that being said, the original game already has a huge selection of units and maps.
Basically, if you can buy some kind of pack that includes the expansions, then there is no reason not to buy it. Otherwise, the original game is fantastic and will keep you entertained for ages.