I think the first thing you would do would be to decide on what kind and what bars you want to use. Figure out what shape (or better yet, what model) of bars you want, and figure out if you need a stem with a different sized clamp diameter.

The next thing is brake levers- you'd need to find some drop bar brake levers.

Because flat bars are different diameter from drop bars (generally speaking), you'll need to figure out how you want to be shifting. You may, or may not be aware that Suntour's Accushift indexing is different from Shimano's SIS indexed (click) shifting. 6 speed is the same, but 7 and 8 have different spacing and cable pulls. If you go with bar end shifters, you'll have to find Accushift compatible shifters, OR friction shifters, OR change your derailleurs to Shimano if you want to use Shimano shifters.

I have not had any luck with pairing Suntour indexed shifters with Shimano rear derailleurs. I've heard of some people having the opposite experience.

This is an experience of figuring out what you like, and how you want to accomplish it.

Good luck!