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Avoid copper like the plague - very, very bad. Yes, some people have had amazing LUCK using it - but I've personally seen disaster several times. Carbon steel is 2nd on the no-go list. OK, maybe galvanized steel should be 2nd - it causes polymerization AND flakes off (so does POR-15 fuel tank 'sealer').

Both of these metals increase the rate of natural degradation of the VO - called polymerization. Together they do it very fast! This is the chicken-skin or fruit roll-up looking stuff you see where VO has 'dried up'. It is in effect how they make oil-based paints, but loooong before you can SEE it, it is forming and beyond a host of other show-stopping qualities will lead to a short VO fuel filter life.

Aluminum and stainless steel are the least reactive metals and SS is generally cost prohibitive.

The tube-in-shell or hose-in-hose heat exchanger you are describing is fine, just use different material. Most of us use aluminum tubing (its cheaper than copper anyway ) for fuel lines and sometimes heat exchangers. For heated fuel lines, we run the tubing inside 3/4" heater hose. On my original conversion, I had a 25ft 'loop' of this style heated lines (called Tube-in Hose or TIH) and that was PLENTY of heat for even the coldest climates. I made a 'loop' by running the return VO line all the way to the VO tank, but instead of dumping this (already hot) oil back into the tank, I tee'd it into the VO 'out' line just before the tank. This way, the 'excess' fuel returned by fuel pressure regulator was simply sucked into the supply line and effectively re-circulated.

I later switched VO pump to an internally-regulated pump (no return to tank), so I added a 16 plate flat plate heat exchanger (FPHE - $125) instead.