First of all, welcome to the Cafe! Good to see a new face around here
As for the deal, I think you'r giving up too much. You're stacked at RB and need help at WR after Dez, who's really a pseudo No. 1. I like Marshall more than Bowe (but it's close) and I think you should be able to get him for DAW straight up. Williams is, and should be, ranked higher than both of those guys in almost all formats, and I don't think you should have to give up your presumptive No. 3 WR and a solid RB just to get another top-end WR2 type.