Triumphs or Nortons. Parts are available for most models, but be prepared for $$$$.
You will also need to be your own mechanic.
My first bike was a 72' Honda 750K. I rode it for 3-4 years before I got a 83' Yamaha Seca 900.
Personally I would never go back to an old bike. They ride like crap (shake the crud out of you) do not Handel well, (more like a bicycle), and require lots of TLC. Something is always going wrong (see the Honda).
The Yamaha was a great bike.
The difference in technology in roughly 10 years was astounding. The Honda had roughly 65 hp, a reported

top end of 120 mph (mine could NOT break 100, 95 was the best) and was scary in the "twisties". Lots of frame flex and the balance of the bike was horrible.

(yes I know the CB 750 is "legendary" for it's time) Even so it was fun to learn on, you can't get in much "trouble" with it. The biggest problem was riding comfort. At 6'3 I was honestly too big for the bike and it gave me a sore back.
Going to the Yamaha we have 97 hp
at the rear wheel. A top end of 140 mph+ a MUCH better braking system, very comfortable on long rides, and handling to match. Just a great all-around sport cruiser.
Another possibility is Moto Guzi, or Ducati, but now you're getting expensive.
Personally, I'd get a mid-late 80's Jap bike that strikes your fancy, and use it to learn about fixing up a bike, before tackling a classic, plus get some miles under your belt. I'd go with a 750cc or bigger range. I'm a firm believer in having plenty of HP.