"Any thoughts on the Fujiwara FKH Carbon"
I have the 240mm. I like it plenty well. It sharpens very easily and takes a fine edge. It is thinner than the Tojiro so it can feel sharper even with the same edge. I don't think it gets a lot of attention because it is a generic carbon, it does not have the edge retention of an Aogami super or the fine grain of a white steel, but for most home users, even those who entertain, it would be hard to tell them apart. I actually got this knife to experiment with carbon steels and now carbon is all I use. I do recommend this knife, though these days I prefer a Japanese style handle.
"Also, aside from D-shaped handles, should being left handed affect my choice?"
There are asymmetric grinds that can have an adverse affect on usability for a lefty. I just looked at the Fujiwara and Kohetsu and they look symmetrical to me, though my eye is not great at seeing grinds.
"If I did purchase the Kehetsu, what else would I need to purchase to get the most out of my investment?"
To keep cost low and return on investment high, look at the Imanishi combo stone, http://www.chefknivestogo.com/imtwosi1kst.html
. This will be a good intro setup and will cost very little. Munky is right, you will want other trinkets to maximize your edge in the long run but learning to use stones and strops and compounds all at once is a recipe for frustration not satisfaction. Get a simple stone set up, when your knife needs sharpening learn to put a good edge on with the stones, then come back and see what the next level looks like.
"Will there be a noticeable difference between the FKH and stainless Fujiwara, other than the upkeep needed? "
Look at forcing a patina, this knife is very reactive, you will want a patina to make upkeep more manageable. The video on the FKH 240mm gyuto page is Mark forcing a patina on a Fujiwara, http://www.chefknivestogo.com/fujiwara2.html