Hi, here's a copy of my Pgh-DC trip report in 2006, and a copy of our Pgh-DC trip report in 2004. (We do it every other year).

The GAP is a great surface, the C&O is a bit rougher (and it doesn't drain well). You'll have no problem with a one-wheel trailer; you'll regret a two-wheel trailer (I did). The C&O is double-track, and the two wheel trailer was a mistake for me.

The WMRT is a great alternative. There's a bike shop and a good restaurant when it goes through town. I'm not aware of any other great alternatives.

Re expenses, somehow I always spend about $40/day in food/gatorade, and $40-$90/day for a room.

May I mention some details? A fundamental question in 2008 is where you start/stop on the western edge. The GAP is not complete to Pgh. Look at the Great_Allegheny_Passage Yahoo group and ask their advice. Personally, I strongly discourage riding from Pgh to McKeesport. McKeesport is a much better place to start/end your adventure. (The trail should be complete in Oct.2008)

Check out the maps on http://www.atatrail.org.
Check out this book: Linking Up: Planning Your Traffic Free Bike Trip Between Pittsburgh and Washington, DC by Mary Shaw, Roy Weil. It's on Amazon for (I think) $5. It's a must-read.

Best meal along the route IMO is The GI Dayroom in Meyersdale PA.