I built a short ladder for my van a few years back out of 4x2 CLS,
Notched the steps into the CLS by 1/4" and screwed through with 4" coach screws worked really well.
I would think you want to paint it with a sealer, to stop water getting into the concrete and cracking it? Something like a bituminous paint for instance. Or a exterior brick paint?
Unless the metal bar is deep 100mm l+ and preferably a L section it's not going to help sagging.
Tbh for the price of £20 for some timber I'd be replacing it!
I'd agree I'd start with a cheap set learn to sharpen them, use them for basic work and then put them in your carrying tool bag and buy a nice set for workshop use.
I like the old blue Stanley ones, good steel from days gone by.
I don't know about spares but it's well made. Motor is easly accessible as are the gears, dust extraction is it's weakness so I built a hopper and 90° soil pipe.