Round or rectangular coolers both work fine. I have a 10-gallon round cooler with a copper manifold (and I use it more than my 15 gallon SS mashtun with a false bottom). Having had both a SS braid and a copper manifold, I prefer the manifold because it takes the abuse of a mash paddle much better and it doesn't float up like a braid can. Over time, the braids tend to suffer, although, they're cheap to replace. Copper is definitely more expensive than CVPC but it's not as easy to scratch and some might say it's more aesthetically pleasing. Plus, your yeasties will like a bit of copper in the wort. Either way, building a manifold is a fun project and there are plenty of examples of pictures and how-to threads on HBT. But the braid will get you started more quickly.
Bargain Fittings is a great source for the bulkhead and valves. For the price difference, I am not sold on 3-piece being better than 1-piece, though. Both can be broken down and cleaned when necessary.