Thank you so much for steering me in the right direction. I had looked at the Quantum Trio, but was concerned it might not have enough power-- it is hard to find any published facts on power level. But I didn't know about the Quantum x5dR which is clearly specified at 400 watt-sec, which seems to me to be enough power for outdoors in bright sun. By the way in this power range I also found the Flashpoint 400 (http://www.adorama.com/FPBPL.html
) which is a low-spec chinese 400 watt-sec system said to be capable of camera mounting, and cost only $178. There are a few others from China in this range listed in eBay, but hard to know how they actually perform.
I had also looked at Metz 76, a handle mount system which is super expensive, but I'm again not sure that this has enough light to be worth it, and it has poor online reviews. I have a couple of Metz 60s, and they don't seem to have all that much more light compared to Nikon SB-900.
So I'm thinking to follow your advice with Quantum x5, if I can find an affordable unit, otherwise go for the nearest chinese knock-off. Again thanks for this valuable advice. If this works, then I will not be forced back to the super expensive Mamiya and Hasselblad MF systems with their (limited focal length) leaf lenses.