pick thickness of the engineered wood planks
Underlayment doesn't settle or compress appreciably, and you usually won't find varying thicknesses of your particular choice of flooring. You'd have to change brands to change thickness. 

Unless you have some specific height need, purchase what suits your fancy. The transition between the new and old floors should accommodate, and a difference in height of 1/8" or 1/4" doesn't cause a problem. 

If you're aiming for perfect level, you may need to install additional subfloor material before the flooring underlayment.