Take some flourocarbon leader (36") and tie a swivel with a surgeons knot (near the middle of the line), then take the trailing ends and tie a bucktail on the longer leg and a orange jig head on the other leg of the hanging leader. Make sure the two legs are not the same length (bucktail longer and jig head about 10 inches above the bucktail)...this creates a umbrella type of movement when you jig the baits up and down. Tip you jig minnow only and the bucktail gets a strip of squid (shedder oil). Hot ticket for fluking.
20 to 25 pound mono. Any more is unecessary. Flounders teeth are for grasping not cutting, even a doormat won't break your line with it's teeth if the drag is set properly. Plus, light line is harder to see and allows the baitfish to swim well, more hits. The hook is best to be a small wire variety. #4 or #2 wide bend or Kahle is good for the minnows at this time of year. The swivel isn't as important, but your basic 70lb barrel swivel at the top is fine. Buying rigs is a waist of cash and you can tie your own that catch more fish any way. Some store bought ones have giant j hooks that deter a hit and some of the line is better suited for tuna!!
Different brands of leader material have different diameter thicknesses. I personally wouldn't use anything less than 30 lb test, unless the situation demanded it. I don't want to lose the fish of a lifetime. My most recent trips we were using 40 lb regualr and 50 lb fluro. Caught fish after fish on either. Use gammi hooks, they're worth it and will add fish to your cooler each trip.
It depends on what type of rig you are tying. If you are just going to make leaders for fishfinder or standard 3 way swivel bottom rigs, then you can use the lighter 20-25 lb mono as underdog stated. But if you are tying a high low type of rig I like 50 lb mono leader material. Its extra stiffness keeps the rig from tangling up.