Afaik you can still use your 8.1 key for win 10, otherwise MS still has a special download for full win10 that activates itself since it is ment for disabled people
g sync isnt worth it since youre porbrably hitting constant 60fps on med-high in 1440p anyway.
https://pcpartpicker.com/product/T8dFf7/dell-monitor-u2515h looks good enough