I restarted my bike passion at 50 on a 10-speed Miyata and in retrospect, it sucked. You won't believe the difference in a modern carbon bike. When we get into higher-end bikes, it becomes a question of how many $100 per ounce kind of thing. You'll be happy with anything under 18# with saddle and pedals. I bought a new ''99 carbon Trek in 2000 and it's still my main ride. What astonished me on my first ride was that i could pedal hard and as long as I kept shifting up, I just went faster and faster. All my power went into the road. That's what to look for. Some bikes are magic. 10 Wheels BMC is one of them.
Yes, a little faster but mostly more fun, easier climbing, easier to ride long distances, and the ability to change components to suit your strengths and weaknesses..
In this situation, I'd buy used. Avoid all the hassles. You'll need to know frame size. You can modify the exact fit quite extensively as long as the frame size is right. I suggest something carbon with compact gearing, 50-34 in front to avoid replacement part issues later.
You can use this calculator:
https://www.competitivecyclist.com/S...ulatorBike.jsp
to get frame size.