17mm vs. 24mm really comes down to which focal length you prefer. Personally, I chose 24mm as the best match for my style. Especially for landscapes, I find it's wide enough. Occasionally, I could use wider for architecture, but that hasn't happened often enough for me to strongly consider the 17mm.

I use the Lee setup. You need the foundation kit and an 82mm adapter ring (the wide angle one is better, and of course you'd get a 77mm one for the 70-200, etc.). The main downside to Lee is availablility - they produce everything in small batches, so often you have to wait for the item you want. The foundation kit seems to be in stock more often now, but as you say, there is often a long wait for the Big Stopper.

Soft vs. hard grad is all about the scene - for a crisp horizon like the ocean or flat land, hard is better; for a horizon broken up by mountains, trees, or buildings, soft is a better choice. I find the 0.9 soft a good first choice. For sunsets/sunrises where the sun is at the horizon, a reverse grad is a better choice - Singh-Ray or HiTech, I'll likely go with the former in a 0.6.

For the CPL, if you're using it alone or with a solid ND like the Big Stopper, you don't need anything extra - the holder can rotate around on the adapter ring to adjust polarization. If you want to combine a grad ND with the CPL, that's when you need more hardware - there are a couple options. First is a second foundation kit and the tandem adapter that allows you to connect them, and then you'd use a 4" square CPL in the outer holder. The second option is the 105mm accessory front thread adapter ring, which would go in the outer slot of the single holder, and you'd screw on a 105mm round CPL (B+W, etc.). The first option is a bit cheaper, but also bulkier.

Hope that helps!

--John