With Veral's book you are on the right track
Lyman's cast bullet book is good. Handloader's book on cast bullets is good
I've got molds made of Aluminium, iron, none made of brass and don't know of any made of steel.
I have no preference between the two materials, a good mold is a good mold. The iron molds will rust, but now days it's relatively easy to prevent that. I use an old ammo can with the dessicant gel to absorb the moisture. The aluminium does not rust but can be damaged with careless handling.
The most consistant bullets will come from quality molds and consistant technique in casting and consistant alloy. I suspect that if you have quality molds, your technique will be more important then the molds.
I've got three RCBS furnaces, one old Lyman and one really old lee all produce good bullets. The downside of the Lee and the Lyman pots is they are prone to spout freezing on cool days, due to being under powered, smaller heating elements.
I've learned to over come this, to a degree, I'll put an ingot mold over the top of the pot to help retain the heat.
I've never owned a lead hardness tester. I use my thumbnail
Jim