Your boned if your using quadratic decay keeping it to 1.

I would just use whatever works for it, I might be tempted to use a "bundle" of directional lights with shadows on (about 6 at random directions) to get an ambiant overall look to it, maybe change the colours a bit to random it up a touch looking at the background, then use another directional light to give the key light (sunlight) to the scene.

If you use quadratic decay it gives nice looking decay, but you will have to overcrank the values to really high values (might be something daft like 2000!)

Have you got the alpha working?

You have to set up the fresnell effect in the hypershade, create a sampler into node and a ramp (remove the place 2d node) then connect the facing ratio to the Vcoord, then make the ramp black/white and then attach this to the refelctivity (or whatever you want to) if you wan to see it working attach it to the colour node of a material to see it working (if should look like its black in the middle of the material and as the surface goes to 90 degrees to the camera it turns white, heres an EG with red and green.