The TX/XS 650 from Yamaha is basically a late 60s Triumph engine, built to Japanese specs. I.E. tighter tolerances, and more reliable with less work than a Trumpet.
Aftermarket is MASSIVE for them as well, and you can still find them sorta cheap if you search.
Honda twins are nice, but 99% of small Hondas are not performance bikes from the factory, so as long as you're ok with that they're hard to beat.
The Honda CB/CL350 twin is one of the most mass produced motorcycles of all time. Parts are so cheap and plentiful its insane. And the aftermarket, performance, and NOS shelves are overflowing to boot.
Dont discount the Yamaha two strokes (mostly the RD line) either. Relativlly cheap, light and fast. Lots of go fast parts for them as well (as long as you avoid the RD200 and RD125).
The Suzuki GRs of the late 70s/early 80s are dead nuts reliable and fun bikes.
Honda baby I4s (cb350four, cb400f, cb500four, cb550four) are TERRIFIC BIKES. Especially the cb400f, which may be the closest thing that Honda made to a production racer (that Joe Public could afford) in the mid 70s.
The CB750 has a huge following, BUT in my humble opinion, Id rather have one of the above Inlines 4s.
Now, If you can afford one...
60s and early 70s Triumphs, Norton and BSAs are the "real deal" as far as period correct BS. I say BS, because if those cats in 60s England couldve had a cb400f, they wouldve sold theyre Bonnies and Gold Stars in a heartbeat. They can be built to be TERRIFIC bikes, but it will take some more time, work and cash than say... A 70s Honda.
That said, the old Brit iron has a class, and spirit like no other machines. They take a little more money to build, but really... You cant take your checkbook with ya when you kick the bucket right?
Another option, if your down with ruffling feathers, is an HD Sportster of any vintage. Itll definitly be the most expensive option. BUUUUT... They can be amazing. The Sportster was the Americans way of trying to steal market shares back from the Brits in the 50s and 60s. If it wasnt for the Japanese basically taking ALL of the shares in the 70s, the Sportys (or derivitaves of...) and Bonnies (Goldies, etc...) were as close to performance bikes that most guys could afford.
Hope that helps a bit and welcome to DTT!