There is nothing special about the line. Plain water grade pipe is fine. You can get yellow coated copper from Mcmaster.com. The yellow identifies it as a gas line and the coating also helps protect the pipe from corrsion and chaffing. You have to strip the yellow coating off before using a fitting. The copper lines are good for several hundred psi and you are using about 5psi. The same type of 37 degree flare fittings are used in aerospace applications all the time. You don't need tape or seals other than a good flare on your tubing. If the flare is split or damaged you need to cut it off and make a new one. Soapy water is great for checking leaks and you need to check all joints after making repairs. Copper and brass are not going to be harmed by the soap but you can wash it off if you like. If you need to use tapered pipe threads (NPT) on something use teflon of whatever color you like and use one layer. More layers and it will bunch up and not seal. Use only brass or stainless steel pipe fittings.
The main reason for the overkill on the lines is in case the regulator fails in which case you have about 100psi to deal with.