Hi. I have a G10 model, which has the same scaler (I believe) and screen anti-reflective coating as the S1 series.
My 50" Panny directly faces two southern-facing windows, and there is an eastern-facing large patio door to the right of it. I was also quite concerned about reflections - which is why I bought the Panny. I suggest that you go to a store and compare these to other TVs. The Samsungs with the matte screen finish have low glare, but the standard glossy screens are horrific (IMO) for reflections. The Panasonics have a nice, muted screen finish that cuts way down on glare and reflections. I've been very pleased with mine.
As for SD material, I've been fine with mine. High definition TV broadcasts and blu-rays are better, of course, but standard definition doesn't look horrible. If you have an upscaling DVD player or are planning on getting a blu-ray player (which will upscale), then it might not be an issue at all when you watch DVDs.