Get a "flip-flop" rear wheel, put a single freewheel on one side and a fixed cog on the other so you can try it both ways.

Re: fixed gear --I'm 61 years old, and having my legs moving all the time isn't a problem. In fact, it keeps the blood moving so they don't get stiff. You can choose whether to pedal hard, or just let the bike pull your feet around, or anywhere in between. Make sure you have at least a front brake so you can control your speed on downhills, and don't be ashamed to take it slow or even walk on uphills.