I would recommend that you think about a hybrid bike first, as the tires are larger & the 26 inch wheels are a bit stronger. Also, you will be more comfortable riding on an upright position. If you like it, use it to loose weight while saving for a decent road bike.

Frame strength is less of an issue than components. As you work down the left hand side of the BD page, the derailleurs improve (strengthen) until you get past the 105s. After that they start to lighten up for racing. Get a bike with 105s if you can, or as far down the page as you can get.

I'm with John Z... I would contact Bikesdirect. I can say that many people like the "roll" of road tires... however "rolling resistence" is not always related to the thinness of the tire... for example a 25C tire rolls easier than a 23C tire because of over all "roundness". So if your quest is the ease of roll don't eliminate some bikes with 25 or 28C tires. You may find a bike like: http://www.bikesdirect.com/products/motobecane/road_bikes/mirage_s.htm
To be perfect. Good luck!

I have no way of knowing that, myself. Although I personally don't know of any existing weight limit, I should think it best to contact the complany for thst information.