I sit 8-9ft away from my 46" Panasonic GT30 plasma and some SD channels can look OK, but many look poor, with some of the worst Freesat channels being almost unwatchable.
Upscaled DVDs usually look fine, although I guess they are normally about as good as SD gets.
There can be a big difference between individual SD Freesat/Freeview channels, presumably due to improvements in recording technology (the age of the program can make a difference) and the difference between broadcast bitrates making some channels look better than others - something like a current episode of Eastenders on BBC1 might look OK for example, but an old episode of Red Dwarf on Dave will look awful.
I'm gutted that you can no longer get 46" 1080p plasmas because I'd struggle to jump to 50" while still watching SD material at my (fixed) viewing distance, no matter how good the upscaling is I suspect. Based on my experience with a 46", I'd guess I'd want to be sat a good few feet further back to watch SD on a 50" screen (so 10-11ft minimum).
Everyone's eyesight is different and everyone has different expectations/standards though - you'll need to demo your chosen set (or a comparable one at least) at your chosen viewing distance to be absolutely sure.