Any flash will work, as long as it has either a PC-sync or hotshoe connection. Probably best not to buy one with a high sync voltage though, I'm not sure if the -II has an electronic flash connection that could be fried.

In the field, you can get away with a large hotshoe flash (I use a Minolta 5600; an SB-800 or similar would be as good - anything that can do about 80Ws to 100Ws) and a mini (60cm) softbox. Indoors though, a nice big monolight or three is nice to have - 300Ws each at absolute minimum, optimally more like 1000Ws if you like shooting on slow film and/or using heavy filters.

There are also intermediate systems: portable, powerful (500Ws) and with large Li-Ion or Pb batteries. Not cheap.

My personal experience with the cheaper Chinese flashes is that they catch fire when powered up for the first time - that was a buying mistake that cost me $800. Cheap chinese radio triggers are excellent value for money though.