Well, since "true" English Wheels have large cast-iron frames, and in most cases haven't been made in any real quantity in forty years or more, the "good" ones tend to be highly sought after, very expensive, and even more costly to ship.
That's not too much of a burden for a shop doing extensive sheetmetal work, and/or a shop with buckets of cash to burn, that's a bit out of the range for most of us home-shop types who might simply want to build one car. Or in some cases, even just make one panel.
The Harbor Freight kits- the yellow one above- is said to be inadequate crap. There are many who have bought it, since they can't even buy the material for that price, and then extensively reworked it and reinforced it. That's an option, as I'm told the wheels, while not truly hardened, are pretty decent for the price, and as long as you don't wheel over chips or grit or weld seams, will do surprisingly well.
There is nothing at all special about the frame. It needs to have the throat clearance sufficient to accept the largest panel you plan to work, and it needs to be rigid enough to not flex greatly as it's bearing down on the sheet.
As casting an iron frame is a bit out of reach for most of us, we're stuck with welding something up out of structural steel. Rectangular tubing is usually the most popular, but I've seen round tubing, pipe, I-beam, channel, and frames fabricated from plate. Some work well, some are too light, and all too many try useless additions like sand or foam to fill the hollows.
The fillers help dampen the noise when you're using the frame with an pneumatic planishing hammer, but are otherwise worthless. They make the frame heavier, but that's about it. Flex between the upper and lower wheels is the only critical issue, and any loose filler cannot and will not help this.
I've seen some people fill their frames with mortar/concrete, but I doubt it helps as much as adding half that weight in additional steel bracing would have.
If you're doing it on the cheap, HF sells just the wheel kit. They're only adequate, but they're inexpensive, and better wheels can be retrofitted later if need be. Then, as somebody said above, design a good strong frame, then at least double it. The frame can't be too strong.
Forget about fillers or dampers, they're just needless weight that doesn't do anything. Use some big casters that can lock, and take the time to make the lower wheel adjustment and quick-release as good as you can make it. That's the key to a wheel's user-friendliness.
Doc.