A roadbike will probably hold the extra weight just fine. For specs, be really sure that you have the eyelets for rack and fenders that you need. You should also be considering what conditions you will be commuting in (rain? snow?) and whether skinny road bike tires will be up to the task or not. Also, adding weight may make the ride a little rougher, which would be another reason to consider a bike that can fit fatter tires, especially if you're going to hit any rough terrain on your commute.

Make certain that the fit is right. You can buy new components later, but you can't change the geometry of the frame (or at least it's not practical).

$300 isn't going to buy a lot if you're only looking at new bikes. I'd do some research on what components and features you think you'll need the most and spend a lot of time bargain hunting. Settling for used will probably get you a lot more if you are careful about what you buy.