Hey, 摄影小菜鸟!
1. Nikon 18mm - 200mm f/3.5-5.6G ED IF AF-S DX VR II Len is a solid all purpose lens and should serve your purposes well. I used both the first and second generation of this lens. It's very sharp in the wide/mid-zoom range but noticeably less sharp at the telephoto side. But most of the your photos are for Web-only, I don't think sharpness will be a big issue. To answer your first question, there are a few disadvantages of the 18-200mm lens compared to the two lens combo.
a. The 18-200mm is heavier than any of the smaller lenses.
b. The 18-200mm is more expensive than the two smaller lenses combined. This is largely due to the higher production cost of the wider zoom range.
However, if I were you, I'd still go for the 18-200mm because of its convenience. Changing lenses in the field can be a real pain in the neck especially for the photojournalistic work you do. Nikon does have a 18-300mm lens, but it costs almost $1000. I doubt if you really need the 300mm for most of your work.
2. Adorama and Cameta are both reputable dealers.
3. You do have the option for get a used first generation 18-200mm for a lower price. Optically it's pretty much the same as its successor. The only down side is its louder motor noise that gets recorded in videos, if you use autofocus in video mode. The second generation is much quieter.
My two cents...:-)
