I’ve made well over a dozen tools for myself and friends. I’ve used mild steel, stainless steel and silver steel for the shafts and can’t see any difference in use with any of them.
Carbide cutters come in several types, flat, negative rake or cupped. Flat and negative rake cutters are normally used flat on the bar. The round cupped cutters can be mounted flat or at an angle to to make them less aggressive. Bars to mount the cutters on can be round, square, hexagonal or flat depending on its use.
Best advice is to study existing tools from top manufacturers like Hunter Tools or Simon Hope and go from there. The best fasteners to hold the cutters are Torx insert screws designed to secure metalwork cutting inserts. You can get them on eBay quite cheaply, the best ones come from Taiwan.
These are a few of the tools I’ve made. My favourite inserts are the 6mm round cupped and 9mm round flat. Any questions ask away and I’ll try to help if I can.