I hear ya about the scared of heights thing, not fun. As to your battery, I have heard good things about ULTRA LIFE, 10 year, smoke alarm battery, U9VL-X: it's a 9v lithium designed for smoke alarms. I wouldn't use it in a combo smoke/co alarm as the co takes a much higher drain and the battery won't last as long.
As others have said, the newer 10 year lithium sealed alarms are pretty good. First Alert and Kiddie alarms are both good, but like light bulbs, some batteries will last the 10 years, others only half that or less. I use the Kiddie's in several apartment buildings I manage and all alarms have made it passed the two year mark 8)
Both companies make battery only, and hard wired with battery backup. The hard wired are capable of being wired in series so that the alarm will go off when others in the series sound off. As to lowering the alarm from ceiling height, don't. Smoke is like water, and chooses the path of least resistance, which is straight up until it encounters the ceiling, then rolls down the walls. A pain, I know, but necessary.
Don't waste your money with a combo Smoke/CO detector for that height. Carbon monoxide is heavier than smoke, and by the time a detector at that height goes off, you are probably already incapacitated. CO detectors should be placed at low heights in rooms for best efficiency.
Good luck!
Edit: Regardless of which battery or alarm you choose, you should still test the alarm at LEAST once a year anyway. I test all of mine twice a year, at each time change in spring and fall. I have had more than a few of the older types fail between test periods, without any indication. Just an FYI...
