The poster who suggested a crosser was exactly right. Thin-tyred racers aren't impossible for someone your weight to ride, but they're a bad idea - the design is optimized people weighing more than 100lbs less than you and the particular circumstances of racing. (and very often give even these riders spine problems.) just trying to get the tyre pressure right for someone of your weight means abusing the hardware your safety relies on. Tyres might be cheap to replace, they're key to braking and turning, and therefore not going under a truck, and i'm doubtful that you'll be able to keep 25mm tyres in the design range for psi without risking pinch flatting, rim damage and sudden blow outs.
A crosser with the right tyres will be more comfortable, brake and turn better, and be less liable to give you a spine injury. Oh - and you'll probably want to get it fitted with wider than standard handlebars as well as 32-45mm high performance road tyres like marathon supremes.
The other thing you should do before ever going near a store is to research bike fit for yourself - google "bike fit calculator" and when there are options choose those that give the most relaxed fit. Then consider buying online from somewhere like bikes direct and having a local mechanic tune and check the bike, because you really can save about half the sticker price this way. Look at information on required tyre width/pressure and weight carried too!
Bottom line: The more you know, the less likely you are to be screwed over - deliberately or by accident. And the further you are from the ordinary customer, the more responsibility you have to take for your own welfware.