Like most new and soon to be fixed gear riders you are over thinking this. I did the same which is why I created the chart to help me think about it better. Since you don't have the exact 42x17 on your bike now, use the chart to find the closest equivalent and if not an exact match use the combo that is higher on the fixed. Keep in mind crank length and wheel size will change your overall ratio as well.

Pretty much all fixed gear riders change gears at some point, so pick something close to what you want that is cheap and relatively easy to obtain, then go from there.

Al