3/16" is standard line going to the wheels. There is no advantage or benefit to using a different diameter, and you'll have to use a bunch of adapter fittings. I don't know where you're going to find brake lines that are made from something different than the original ones. If you use some non-standard tubing you run the risk of getting lawyers and insurance investigators involved. I don't pretend to know anything about the composition of standard brake line but I can share that copper tubing will not hold up to normal brake fluid pressure. You can easily reach over 2000 psi. Copper tubing will explode on power steering systems that only reach 1100 psi.
Copper tubing is normally only flared for plumbing uses. Brake lines need double flares to seal against their seats. Copper is softer than steel and will crush when trying to make a double-flare. Harder metals will crack on the final step or the first step of the double flare won't form properly. The tube will kink instead of making a nice symmetrical bulge.
If you're worried about ocean salt, which is no more destructive than driving in winter in Wisconsin, the road salt capital of the world, spray rust-proofing on the lines once they're bolted in place, especially around the fittings behind the doors. That's where salt tends to collect, at least from road salt. Unless you accidentally drown your truck, in which case you have bigger problems than rusty brake lines, salt water shouldn't reach any lines or fittings road salt wouldn't reach.
I sprayed 14 quarts of rust proofing on my '93 Dynasty before it ever left the dealer's parking lot, and there is not a spot of rust on that car anywhere. Of course it only has 4,200 miles and was never driven in winter, but I also have an '88 Grand Caravan that is still driven daily in winter. It has never been parked in a garage since the day it was new. Only three brake lines have needed to be replaced about five years ago. The rest of the van is leaving me, one crumb of rust at a time, but all the other steel lines still look fine. What I'm saying is, stick with standard, pre-manufactured lines with the fittings already on them. You won't have to fight making the double flares and hope they'll seal.
I should probably mention too, for the benefit of anyone else reading this and contemplating using non-standard parts, compression fittings are absolutely never allowed on high-pressure brake lines. Nothing but double flares or iso flares, as it came from the factory. Compression fittings WILL blow off since they're not designed for such high pressures. That's why you never see them on construction equipment. This is where my "lawyer and insurance investigator" comment comes from. Once they find modifications like that, they will look that much harder to see what else has been changed. They are great at convincing juries you were partly to blame for the crash when the other guy ran the red light because you were less able to avoid the crash.
Now, ... All that said, if you are looking at using some kind of higher-quality line that is sold commercially for use on brake systems, I simply think you're going overboard unless you've replaced the same lines multiple times before. I've been a brake system expert for 28 years and have never run into anything other than new lines in common, standard lengths with the fittings already on them, or factory-made lines for exact replacements for the specific vehicle. You can buy the tubing in bulk rolls but once it has been coiled up like that, the double flares always come out off-center. Standard-length, pre-made lines are fast and easy.