I know you can find a bunch of NOS Campy Veloce and Centaur stuff from recent years. SRAM and Shimano would be fine too. Go for 105 for Shimano - slightly heavier than Ultegra (or go for Ultegra 6600SL because it would look fantastic on an older steel bike).

Sram starts with apex and rival with the main differences between them being more alloy in the apex and more carbon in the rival. Both can be used with 11-32 cassettes as long as you get a med. cage and are roughly equivalent with 105.

That's some starter info and I could be a lot more detailed, but I encourage you to go shift some levers first and discover what you like