We dont even know what kind of plane he is building.

Go take a picture or describe it to us. It all depends on how big the rudder is, not how big the plane is.

Here is a simple answere to servos.

$50, you should get 5645's or 945's or maybe 645's for $30

but a way better servo fo raround $70 is the 5945.

Never ever put two 5945 or comparable servos in line, it is pointless unless you already have them, One $100 5955 or 8611 will give you more power for less money.

At the most you should need 2 5955's.