D B i would go with the YS 120, it will give you bags of power and it has around the same or possibly slightly more power than the saito 150 any way. As mv-warbird says a 16x6W or standard 16x6 apc would be ideal.

if you are wanting to make it a crazy 3D/freestyle machine scrap the thought of putting a 6V 2700 mah (presuming you are thinking about a nicd or nimh) battery in it, it will be way too heavy. I would go with a fromeco 2600 mah li-fe from DA, you will also have to pick up a suitable regulator to bring the voltage down to 6v, DA markets these too.

We went down this path with our funtana x 100 and replaced a 1400 mah 6V nicd with the set up above and the lithium was still lighter!!!

servo wise, Hitec 6965 digitals are perfect for extreme 3d on you ailerons, they have torque and are extremely fast. The rudder and elevator are the surfaces that are working really hard in a 3D workout and need plenty of drive, Generally i like to keep all the servos powering the tail surfaces the same so when you are doing very quick rolls in a rolling circut no servos are falling behind the other surface.

if you wan't crazy 120 oz/inch servos are a must at the back

hope this helps