I use a 10 gal corney keg for 5 gal batches. It works great. That amount of excess headspace isn't a problem. It's not too much, it's great actually.
Sanitation, I can't get my fat arms in a corney any better than I could get them into a Sankey. PBW cleans it well, either a cold solution overnight, or a hot solution for an hour.
Closed fermentation is the way to go. Are you planning on closed transfers under pressure to you keg? I don't regret the investment I made for this setup one bit. It's so much nicer than plastic or glass.