off topic for HN, but:

Why do you want a telephoto lens? Do you shoot mostly wildlife?

If you don't shoot wildlife or have a specific use in mind - skip the telephoto and go for a prime lens.

A prime lens doesn't zoom, but they usually better quality as you'll get a much bigger aperture.

Definitely own the 50mm 1.8 lens, and be comfortable shooting it. That is where I'm at in photography. The 50mm 1.8 is cheap, < $200, so you have no reason not to own that.

Good luck.