Connecting non Apple monitor to Macbook Pro
Does connecting a monitor to MacBook Pro via display adapter lower the
  refresh rate,


Not necessarily.  It will only do so if you get an adapter/cable that doesn't conform to the latest specifications (i.e. it doesn't support 4K at 60Hz)

The best place to find what the MacBook Pro will support with respect to an external display is directly from Apple:


  Simultaneously supports full native resolution on the built-in display
  at millions of colors and:
  
  
  One display with 5120-by-2880 resolution at 60Hz at over a billion colors
  Up to two displays with 4096-by-2304 resolution at 60Hz at millions of colors
  Up to two displays with 3840-by-2160 resolution at 60Hz at over a billion colors
  


As for what monitor to get, the best advice is to get one that a) works with you and your workflow and b) has a DisplayPort/mini DisplayPort interface.  This way, you don't have to convert from the DisplayPort signal that is natively on your MacBook Pro.  All you will need is a USB-C to DisplayPort cable to connect