Does 6 and above need a Remake?
Rance VI (aka 06) is fine.
Its a dungeon crawler with not bad graphics from 1990 like rance I to IV.
So yeah, 06 its fully playable and fun to play if you like the dungeon crawler stuff. And my opinion its where the plot of the saga start.
You can figure all the plot just by playing 3, 6 ,7 ,8 ,9 and 10. (special mention of 4 just for the mague sect and warlods)
If you want more than just the core plot of the saga. Then play all of them because all of them add new char, continue their story and progress the characterization of every char.
The games that really need a remake is 4 and Kichikuou.