Though I am far from retirement age, I have been thinking about it for a while. Ideally, I would choose some hilltop location in Pashupatinagar, but the need for healthcare in old age may be a sticking issue there. So, surrounding areas near Kathmandu, like Luvu, Budhanilkantha, Bhainsepati could be do-able. However, if I decide to go back, I would probably settle in Dhobikhola area between Kapan and Golfutar. I envy your decision to retire in Nepal. Good luck.