You need first of all to check the fibre of your garments. Cotton takes dye very well - polyester not at all. Check the info on the dylon website for viscose etc.
Secondly, even cotton garments or cotton-rich garments are often sen with polyester thread, which won't take dye - so you can end up with white stitching on a black garment - can be good, can be bad. But as long as you know.
I often use machine dyes and there may be some staining to debris left in the washer, but no damage to the machine or to subsequent washes. The staining is a bit like those disclosing tablets you use on your teeth - it shows where the grot is lurking.
The salt you need is the cheapest big packets of cooking salt from any cheap supermarket - the stuff they sell alongside the dyes is the same ina fancy packet. BTW cooking salt is just NaCl - table salt has minerals added to keep it from clumping. No difference to your dyeing but cooking salt may be cheaper.
First time you wash the clothes after wearing put the clothes in separately as a precaution.