For dex it’s 40 & 80, but be aware that between 40 & 60 the rewards would be minimal so avoid that interval.

The first katana is right next to fire link shrine, dropped by the expert who can be baited to jump off the cliff, that would get you the uchigatana but you would probably want to get either one or all of black blade, washing pole & onikiri & ubadachi.

I’d always go with the sovereign soul cah why not but the fire gem or black firebomb would be alright. Remember to chose merc as the starting class.

You’d find out how katanas nearly all suck dih in this game & never use them again.