Hi,
After looking and thinking for a bit last year, we went with the RuXXac folding hand cart for the heavy bits (tanks, weights). We have XS Scuba wheeled mesh duffels for the rest of the stuff (these are terrific).
They hold an incredible amount of weight for the size and cart weight (125kg or 275 lbs!), fold flat for transport, roll well on good wheels. I have the long footplate version, takes 2 tanks and weights just fine. You will want to add a good strap to secure the tanks to the uprights.
For convenience, I got one for myself and one for my wife/buddy, though it is possible to load 4 tanks and all the weights on a single cart it does get a bit unwieldy.
The carts have been enormously useful in normal life, too ;-).
This is like the first folding cart on the burgeeshoppe link, but that looks like it might be a knock-off, not an original RuXXac. FWIW, we've had several of both real RuXXacs and knock-offs at work: RuXXacs going strong after much abuse, knock-offs are trashed. I'd look for the real RuXXac. You can usually tell a knock-off by the lower weight capacity.
I got mine throught the Container Store (I was impatient to have it for a charter coming up quick), but there were many online vendors as well. For some reason, I can only find UK vendors with a quick search this morning, e.g.:
http://www.liftruck.co.uk/ruxxac-carts.html
They come in several models, some specifically for marine use (I have seen the regular carts by boats, though, they seem to last fine).
Hope that helps,
Cheers