For 1/2a I use braided spiderwire -- 20lb test / 8lb diameter. This means the line is the same diameter as 8lb monofiliment but is as strong as 20lb monofiliment. Also, the braided line is more flexable than monofiliment. I usually use a clinch knot or variation of a clinch knot and leave a short tag of line so I can tell if the knot is slipping. So far no slips. Some people prefer the Palomar knot. (google fishing knots and find all sorts of interesting and effective knots) Some people put a drop of glue on the Knot, but I usually don't. I have used the little spools of thread that used to come with a lot of 1/2a kits and handles, but it is so stretchy it is sort of like using a rubber band.
I usually use 45' lines. With a slower engine/plane combo 35' may be ok. With .061 engines, you may need longer lines.
I have no experience with metal lines on 1/2a, but I'm sure it is a good option.