I purchase step-up rings anywhere I can find the odd sizes I need at a fair price. I think one is branded Tiffen, schneider, and a few unmarked. I haven't found much variance in terms of quality... the filter on the other hand really matters. The more expensive ones, such as Heliopan, are made from Brass instead of cheaper alloys of Aluminum. For some of my smaller lenses, I have to step up twice. In my case, I step all my smaller lenses to 55mm and then use a 55-77mm ring. For anything larger, single step up ring to 77mm. Makes for quickly switching the filter rather easy. For really wide focal lengths, I avoid stacking as much as possible but they generally have greater than 55mm filter size.
Most of Canon's L-lenses are 77mm filter sizes too...
I haven't used Lee filters but I have seen their system in person. The holders are a better quality than Cokin. I haven't found too much need for graduated filters but yes.. you'll want to get square glass ones so you can slide them up and down for more control.