Drills, driving bits and drill bits are probably the most frequently used tools by homeowners and DIYers. Take a few minutes to learn the different types and what you really need.
As for hammer drills, some combo standard/hammer drills are available in cordless varieties. They can be virtually identical to a standard drill, but have one higher setting past the normal drill torque setting, and it conveniently looks like a hammer.
What makes a hammer drill a hammer drill is its ability to also pound/vibrate as it drills. The cordless option will get you by for a lot of applications, such as installing masonry brackets in brick walls, but for heavy-duty jobs, such as attaching support brackets to a patio for railings or posts, you'll want a corded hammer drill. Although for as often as you'll need one, you may be better off renting rather than buying.
In addition to your 18-volt cordless drill, you should also keep on hand a corded drill for a couple of reasons. One, if you're working on a project where you're switching back and forth between drilling and driving, you can save time by not switching out bits and save battery life by using the corded drill for drilling and the cordless drill for driving. Two, if you're working all day there's a good chance you'll outwork your drill battery and have forgotten to keep a backup battery charged.