Yep, you definitely want all steel and fully welded. Bolts just create pivot points and everything comes out of square. I've been doing this with stock wheelbarrows; get rid of the wood and fully weld all the "framework". Flat free tire is a must. Post #25:
http://www.garagejournal.com/forum/showthread.php?t=211676
Exactly!! We never did much wheelbarrow work since we used equipment to move stuff around, but obviously there were times it was needed. Got a basic steel wheelbarrow, once handles cracked from loading too heavy, bought the steel handles for them, removed the angled wood shims and used steel, welded it all together. Also bought a run flat/solid rubber tire for it- pretty much indestructible now since it's all steel, welded, and no way for the tire to ever go flat again
So Start with a basic steel model, run it into the ground, and when it comes time to upgrade the worn parts 5,6,7,8,etc...yrs later, give it the heavy duty makeover

Might even consider a unit with the dual front tires for stability purposes, I've thought about it myself, just haven't dne it yet...but when when I do, I'll just buy another run flat, buy a piece of steel rod that's longer for the axle and make the conversion kit myself vs paying for a kit, simply because there's nothing to them.