Here is my take on your questions.

1) Think about the type of pavement and path you are going to ride. At 300 lbs you are probably on the margin for a road bike. A hybrid with 32 mm tires and sturdy wheels, or a mountain bike with pavement tires is probably a good place to start.

2) Road bikes are generally set up for 28 mm maximum tires, you may get lucky and find something where a 32 mm wheel and tire will fit, but normally road bikes are running 23 mm tires, which can be a bit of a problem, unless you get very good (expensive) parts. Road bikes are generally set up for minimum weight and maximum speed, and are comparatively not as strong as a hybrid or mountain bike. If you are interested in a road bike, you should look for something in the touring line with relaxed geometry. 32 to 36 spoke wheels at least. Surly Pacer (similar to the cross check, but with Brifters) may be something to look at, and there are aluminum models that will work.

3) Hybrids and MTB's have index shifting on the bars, which are less expensive than the expensive brifters used on road bikes. So you get a little better set up (brakes, derailleurs, crank etc.) with the Hybrids.

4) If you have hills, you will need to be sensitive to gearing. A compact double or a triple is much more forgiving when hills are encountered. 28 or 32 teeth on the rear is the minimum, and you should look for a long cage derailleur to accommodate the chain length.

5) When you get to longer distances (greater than 20 miles for most), road bars offer a greater variety of hand positions and are more comfortable as your hands are not outside of your body.

6) Most road bike saddles are inappropriate for clydes, and you can probably figure on changing it. Not that you need a cruiser, spring loaded contraption, but something with a bit of support width and padding is more comfortable for me.

I started out with a road bike, tried the flat bars, eventually bought a relaxed geometry specialized sequoia, but now have a real road bike, with a modified headset (25 degree stem), to keep the geometry similar to the sequoia, I actually taped it and to my surprise, it is within 1/2".

There is a difference between a touring road bike, and a "race" road bike, as you get better and lighter, the touring style becomes less attractive.

You should be able to find something in your price range.