Can you lower the seatpost at all? If you can do this, you can set up the saddle to the lowest possible position, then raise your existing stem to a position which feels comfortable relative to your current (lower) placement of the saddle. Then you can see how the handlebar you already have feels... perhaps either too close or too far... and then order accordingly.

Later, when you raise both the saddle and stem they will retain basically the same angles. So, the distances will remain about the same. Of course, you will ALSO need to use the same handlebar which you intend to use later, because a different style bar can very dramatically alter the overall reach - just as much as the stem extension.