With mechanical brakes, the work rate increases as pedaling speed increases. Thus, you can make the workout harder by either adjusting a resistance knob or pedaling faster. For a mechanically braked bike, make sure you can easily adjust resistance with just the twist of a knob. Avoid bikes that add resistance with rubber pincers that grip the wheel, because the result is often a jerky ride. Look instead for bikes that use either a friction belt or wind resistance.
Some models offer arm cranking.
Racing handle bars, which require bending forward quite a bit, may feel uncomfortable for casual bikers. Set the seat height so that your knee is bent only slightly with the pedal at its lowest point. Use pedal clips, which allow your legs to pull as well as push the pedals. Whatever the reason, consider this concern before you buy. The latter offers a more natural feel, as do electric rowers.