I agree, if you're not going to be bombing down root and rock-covered dirt hills, you do not need rear suspension. Look for a hardtail bike. If you're going to be riding mostly on streets, then don't get knobby tires. Get nice, smooth slick tires. They don't have to be super-thin road bike tires, just smooth.

You may be surprised at the low prices some bike shops charge for their bikes. I definitely encourage you to visit a few bike shops, and ask about used bikes.

You'd also be surprised at how many bikes simply get chucked on the side of the road on garbage day! I plucked a perfectly good 80s 10-speed road bike from the trash a few months ago. A bike you find in the garbage should be thoroughly inspected for damage. It may also need a new chain (almost always) and maybe new tires or other components, but these will usually cost you less than a new bike.

I also recommend you check Craigslist (online classifieds) and Freecycle (classifieds where everything is free - seriously!) for deals.