I'm struggling with this as well, father in law is a plumber and will be helping with the install, scheduled for tomorrow. He says use black pipe for the interconnects, and that I don't need an expansion tank as there's no fresh water source. Lots of things that are in the face of what I've been reading, but then I'm not the plumber here.
The purpose of the expansion tank has nothing to do with a fresh water source. You have a closed system as it heats up, the water expands, without somewhere for it to go, the pressure will rise, possibly beyond what the pipes are rated for.
The use of copper or steel in the system is largely a preference - people like copper for its corrosion resistance, but it is costly. The purpose of the oxygen barrier pipe is to prevent oxygen from being replenished in the system and causing iron in the pipes, valves, pumps, boilers to corrode- so if you are using a barrier pipe, it is safe to use steel and iron components in your system.
Most systems have a hodge podge of plastic, iron, steel, stainless, copper, brass, aluminum ...components anyway - about all that differs is the ratio.