I'll assume you'll be practicing Basic routines and hence probably might not even notice the extra precision of digitals. If you set up the linkages for strictly Imac you will have significant mechanical advantage over the same throws on a 3D set-up. In other words 1/2" elevator deflection at full servo throw needs a lot less torque than the same 1/2" using only a 1/4 of the servo movement. If you want to go cheap use 645's on 6 volts or something similar (stay away from plastic gears) but, the money spent on digitals now will pay off down the road when you move them into a bigger plane.