I was once wondering where did Watsuki-sensei get the name "Kenshin", then as I read the tankuobon no 5 (Yutaro section), Kenshin was telling about another guy named Kenshin, and he (the other Kenshin) was actually a true hero in Sengoku Jidai, the age of war. When I red a book about Bushido, I found a story about him. Kenshin Uesugi was a real man at heart and soul, though he fought against Takeda Shingen, he cried aloud when he heard about his 'beloved' enemy's death (just like Kenshin's feeling to his old adversary, Shinsengumi). One of his most noble deed is when Prince of Houjou wanted to weaken Shingen's people by blocking any transportations which carried salt to Shingen's territory. Shingen lived in mountain range far away from the sea, that's why they really dependent on salt from Houjou, Tokaido.
When Kenshin heard about the dilemma which his enemy endured, he tried the best he could to get enough salt from his own region and sent it to Shingen. Then he wrote a letter to Shingen, that's in his personal opinion, what the Prince of Houjou had done to them is very contemptible. Although Kenshin was in a war state against Shingen, he wants to fight fairly. Furthermore he added, saying " I do not fight with salt, but I fight with sword". That's also what Himura Kenshin did by giving Tsukayama Yutaro (who was at that moment, his enemy's fellow) onigiri, a tasty Japanese rice-cake. I think basically Kenshin inherited Kenshin Uesugi's spirit, they both were pretty gentle swordsman, desu yo? ^_^.

But about the model, Watsuki-sensei said that it was taken from one of the greatest hitokiri (assassins) in the Bakumatsu namely Kawakami Genzai. When Watsuki-sensei wrote Rurouni Kenshin for the very first time in Shonen Jump Magazine, he wrote it as Kawakami Genzai instead of Himura Kenshin. I have a profile of him in the Ishin Shishi's Hitokiri along with 3 other famous hitokiri in Bakumatsu. Just surf there to know the details of him and his comrades.