I think the concern about using containers that had lye in it for food is related to the possibility that some lye residue would remain, effectively poisoning the food, and thus the eater.
I keep the lye containers completely separate, including the plastic container I mix the lye and water in, because it is the only place lye can be found. I bought lard from a butcher shop and yeah! it was made of number 5 plastic. I have a stainless steel bowl I mix lye into the oil. I also found cottage cheese and other things I buy come in number 5 plastic. I clean those and rather than throw them in the recycling bin, I use them in making a mise en place for soap.
While I think many people make way way too much of the danger of using containers that once held soap batter, and I agree that with proper cleaning a stainless steel container used to make soap batter could be used for a salad, I try to follow a policy of could not versus would not. I would not be so sloppy in cleaning a bowl that I would make myself sick with soap batter residue, but if I never use it for food, I COULD NOT make myself sick with soap batter residue if I keep them separate.
I bought most of my soap making utensils at a dollar store, bamboo mixing spoons and a bamboo cutting board for a buck, a nice chef's knife for cutting soap blocks for another dollar, and huge stainless steel mixing bowl for, um, $4 I think. The only thing I might be tempted to use elsewhere is the stick mixer. I bought a cheap one that gets hot when I use it, but hey, it works. I don't have a need to use any of my cheapo soap making things in my kitchen, as a result.
I'm not making soap for sale - I'm making rustic man soap for my family and me. I found I can do it on the cheap, too.