Just a few thoughts. Before the sun's full on the water a full floater and a purple fly of some sort a bit below the surface. Once the suns full on, shift to a deeply sunk fly off a sink tip. One other thing (as you're new to the Lower river) is get a copy of BLM's 'mile by mile' river map. You can get a (simi) water proof one for ten bucks ... or turn your printer on and get same on line.
The maps cover all of the camping areas, what's there (fresh water is few and far between) in the way of camping, bathrooms (all clean as can be), shade, etc. The really good thing (above the ton of info) is these map sheets are done to scale so 'river mile 5' really is at river mile 5.
As for fly choices, many will do, but the 'go to colour' will be purple. You don't need really long leaders off a 2-hander, 9 foot is quite enough. The only other 'toss up' is the amount of silt that the White River is putting into the D; this can make fishing 'interesting.'
Fred