Having purchased this lens myself a few weeks ago many people were suggesting new bags, tripods, tripod heads, etc... Once I got the lens, I needed nothing! I hand-hold it primarily, so no need to upgrade the foot or anything like that. My Manfrotto ball head supports the lens just fine as-is if my tripod is used.
My Lowepro 400AW bag loads my 300 II perfectly when attached to the body (plus 1.4x III extender) as long as the hood is reversed. Along with 2-3 other lenses on top of it. Very happy about that!
So once I got the lens, that is all I needed. I guess if you want a good support system or something you would, if you don't have already, but the lens is really not bad to lug around. I used it as my walk-around on my recent trip to Florida at the amusement parks and at times hand-held it for 6+ hours straight without any issues.
If anything, I would just recommend a good strap... I like the op-tech ones, they are comfy when supporting larger body/lens combos. FWIW, I am using a 5D3+grip.