Awesome game, but much like old Palladium and FASA game titles, you have to buy the 20+ supplements to really expand the campaign world. T:2000 (as we referred to it back in high-school) is set in post WWIII Europe, and though this world/setting is richly described and inspired, the extra details make it pretty apparent how lack-luster the remaining parts of the globe are covered in the core rules book. While there are infinite campaign possibilities in just the ruins of Europe, your players may grow tired of foraging for food while fending off any remaining Soviets/random marauder groups with the Polish villagers they managed to befriend despite not knowing a single word of Polish.
It can get expensive by the time you purchase the core rules book, MERC: 2000 (an essential must-have if you want to trade your post-WWIII campaign for a more current-events real world campaign), and 1-3 of the 2nd Edition equipment/vehicles/other global regions supplements. Each supplement is very detailed and each will greatly expand your campaign, but they're pretty specific so there's a select few I reccommend purchasing before others.
Between me and the other regular GM, we probably owned nearly 70% of the sourcebooks (many in both 1st and 2nd editions) and several of the mission books as well. We grew tired of T:2000 about the same time that GDW went under, and switched to Shadowrun (an equally expensive game that also went under when FASA collapsed, but has been resurrected by WizKids and FanPro).
The absolute essentials (listed in the order you should get them):
1, 2, & 3) Twilight: 2000 and MERC: 2000 2ND Edition rules books (buy them at the same time, trust me you'll want both). Go ahead an grab the Referee's screen too, it's pretty helpful since most of the important charts you need are printed in bizarre places in the rules books and there's no index to look them up.
4 & 5) Small Arms Guide and Heavy Weapons Handbook 2nd edition sourcebooks (be sure to get the 2nd ed. HW Handbook, not the 1st ed. HW Guide). Unless you want to be limited to the 20+ firearms described in the core rules, you'll need both these books to properly outfit your players/NPC's/the enemy for both T:2000 and MERC.
6 & 7) Special Operations and Gazeteer 2nd ed. sourcebooks. Spec Ops has advanced rules (suppressing fire, Delta Force character class, HALO parachutes) and gives details on how to run elite commando style missions properly so they don't end up like a Chuck Norris laugh-fest, and the Gazeteer greatly expands the MERC universe and has a bunch of campaign settings/missions included.
8, 9, & 10) Mission books Going Home, Red Star/Lone Star, and sourcebook Howling Wilderness. Going Home is basically how the players get out of Europe, Red Star/Lone Star lets you re-enact the old West in the post WWIII Texas frontier, and Howling Wilderness is the books detailing North America with info vital for running both of these excellent missions.
All the remaining sourcebooks are really to your own tastes as they cover NATO vehicles, Soviet vehicles, and/or specific regions of the globe. If you don't feel like getting bogged down in vehicle to vehicle combat, or particularly care to run missions in the SE Asian Pacific, then pass. The core rules books has enough tanks/boats/planes to keep you busy.