I have a pair of these:
Nashbar Euro Compact Panniers
The Ortlieb bags you're looking at are a lot fancier, and look easier to handle off the bike. The ones I use are cheap and hold lots of stuff. I sometimes have lots of stuff. I'm not sure you'll fit everything into one of those bags unless you can either leave a towel at destination or at least get one of the super-compact camp towels.
I also have one of
Nashbar's Garment Bags for moving work clothes back and forth - I'm not lucky enough to work as casual as you describe. I don't take that every day though, even during the season I bike almost every day. (I'm a few times a week winter bike commuter)
Your maintenance kit will absolutely fit in a seat bag. Lights I usually bring inside with me. In the winter, I shove them inside the pocket of my jacket. Summer, they usually go into the pocket of the pannier that the bike lock came out of.
I usually only carry one spare tube and a patch kit - I hate to keep myself to only 2 fixes, but like being able to quickly switch the first one.