The biggest down side to buying any bike off the internet is that you need to assemble it, or have room in your budget to pay a shop $100-200 to do it for you. Aside from that, the bike will indeed have a low resale value and the lifetime of the low end components are limited if you start putting on a decent number of miles. Also, at your weight, there is a possibility of needing stronger wheels no matter what bike you get (note, I know someone who started out heavier than you and did fine on weaker wheels than that Dawes, so there are exceptions).