The heat source , equipment and so forth is pretty much the same stuff you could be using for cast bullets.
I use the good known quality lead for bullets and the crappy nasty range sweepings go into jig heads and other fishing sinkers.
Years ago I helped a guy pour a keel for a sailboat he was building . Think of a cast iron bathtub up on blocks with a shit load of firewood under it (with blowers) and literally tons of lead that had to be lugged around . Worked my ass off that hot day
There is money to be made in sinkers , more so if you are near any salt water but the key is how good you are at grabbing up scrap lead.
If you work hard at it and develop some sources that will feed you a steady stream of scrap it can be a cash cow.
Hard to satisfy anything but personal use with bullets as quality of the scrap is critical and the high volume production equipment is expensive but with most sinkers lead quality is not at all critical and low tech is the rule