300 ws is more than ample for outdoors - figure that most shoe-mount flashes are at most 150 w/s. As to the mounting ring, elinchrom vs. say, Bowens S-mount vs. Profoto really doesn't matter if you've not already bought into one system. All of them are available, and for example, Calumet makes a universal speed ring you can use to mount your softboxes or other modifiers, and if you ever change systems in the future, all you need is a single new mount adapter. The Paul C. Buff stuff requires their own unique proprietary mount, IIRC, and you can't buy adapters from other folks.