Lots of good info here already, let me just add my experience for good measure.
I consider myself a beginner (with lots of experience), but I have learned some stuff along the way that I want to share.
I started on a Yamaha (YTS32, I still have it as a backup tenor) and played that for over 10 years while taking lessons. About 4 or 5 years ago I got myself a 10m from 1963, I loved the huge, colourful sound that I could get out of that horn.
I am convinced however that, say 2 years prior to buying that horn, I would not have been able to get that sound out of that horn. I just didn't have the breath support, embouchure, and other technical abilities to get that sound. So don't rush into buying gear to get a certain sound, practice will deliver more results.
After a couple of years trying to get to grips with that horn, I went to London to try out some modern horns that might get me a similar sound, but with modern ergonomics. The key layout and ergonomics on the 10m are quite different to the Yamaha that I had started on. But the change is manageable. But my bari also has the modern key layout, and I still feel more comfortable with that layout.
I found 2 horns that came very close to the sound that I wanted, the Rampone & Canzanni R1 Jazz and the Eastman 52’nd street (with the M-neck). I also tried the TJ-signature (very curious about that one because Stephen has been praising it so much), but that sound was more along the lines of that Yamaha than the Conn, not what I was looking for.
So if I were to get myself a tenor now, I would be going for a Rampone. Based on my (very limited) experience with that brand in London, I got myself a Rampone curved soprano that I am taking lessons on right now. I like it very much.
So I would conclude, don’t rush into buying a new (or second hand) horn. Try lot’s of horns, by all means, but don’t feel compelled to buy any of them as long as they don’t grab you……