Old car drive-shafts….
Well, you can try them! You'd certainly be using high-tensile steel but it could have been heat-treated and if so is basically un-machinable even with a carbide tool. I have tried…..
Also note that if you intend welding the assembly, many steels are averse to anything except whatever rods might be made for that grade, if at all.
And if it's EN1a (lead-bearing, free-cutting), yes, it's lovely to turn but is not made to be welded. Whilst you can "hot-melt glue" it with ordinary mild-steel rods, you can't guarantee the weld won't fail in service.
Cast-iron items not intended for any but very basic finish-machining if at all, such as weights and architectural parts. are another gamble. You can be lucky and find something that gives you the quality you want, but many such items are quite crudely made and can be full of blow-holes and inclusions. And if chill-cast, like old sash-weights, be practically un-machinable. Yes, I have tried that, too…
On the other hand cast iron extracted from a piece of scrapped machinery, is likely to be of very good quality. I once made some parts from fragments of cast-iron gleaned from a motor the scrap-dealer obligingly roughed down with a sledgehammer for me!
Old printers and scanners are a good source of small-diameter ground bar and a few other goodies, and they also contain potentially useful circlips, self-tapping screws, etc.