Not sure if this would sway anyone’s recommendations but I’ll be buying a used bike. Most I’m looking at are under 10000 miles or so
Guzzis and Boxer engines make valve adjustment an easy 20minute mid riide lunch break type operation. But those are full sized liter+ bikes.
The boxer engines are easily 100k plus motors and have often reached 400k plus.
I don't know as much about the smaller motors, but they are typically v or inline configurations, so have less accessible heads.
To sock puppets advice I will add look at the brake lines. Some of the old rubber ones tend to degrade... replacing with reinforced steel braided line is not difficult. A few companies sell upgrade kits.
