VFD= variable frequency drive, basically adjusts the power being sent to an electric motor, this adjusts the speed.

This are interesting concepts, but I think you can make one better.

Yes, triangles are the strongest shape, this is why most structures that utilize a vertical rectangle has gussets in the corners. Welding is usually preferred and a quick way to a stable structure. That's not to say you can't make a stable bolted structure, but it isn't always easy. There's inherent "slop" in each and every bolted connection, so that you can slide the bolt through the hole. Just something to bear in mind in the development and construction phases. If it's wobbly you at least have an idea of the issue, so that you can develop a proper solution

I don't see why they wouldn't. Recycling places pay by the pound and companies usually stock up until they have enough to pay for transport at a minimum. You would, at the very most, be swiping 15-20 pounds of steel. That's probably pennies. I'd keep a lookout for anything in the 1-1/2" to 2" range. Thicker than what you need structurally speaking, but give you enough wiggle room for alignment, however you adhere the pieces to one another.

As far as horsepower is concern, it shouldn't be too much of an issue. I'm assuming you're not going to use this to cut a few hundred pressure treated 2x4's? If you can produce 1/4 to 1/2 horsepower for small scale with a sharp blade, that should be sufficient.

I will think on this further, definitely interesting