Hi

I don't know whether you will be flying JR or Futaba but I'll give you an idea on Futaba servos.The order of importance of the servos is as follows:1)Tail rotor 2)Collective 3)Aileron/Elevator 4)Throttle.
The tail doesn't have to have a digital servo but if you get a gyro i.e Futaba GY401 with heading-lock it is going to allow it to run at its best.One can buy the gyro with its servo S9254 which is a very good combination.
Regarding the collective servo you don't have to have a digital servo, a standard S3001 or S-148 will work but it does lack a bit of torque.I reckon you should have at least a 5kg.cm servo for the collective especially if it is the 50 size heli.The servo doesn't have to be digital or coreless but they will more precise and responsive.If you're a beginner it is not always good to start with fast servos but it will save you from having to change them as you become more experienced.The problem with putting a high speed digital servo on the collective is that if the throttle one is not equally fast it will lag behind the collective but this you won't notice unless you are doing extreme 3D.The Aileron/Elevator servo can also be standard servos.It all depends on how much you can spend and whether you think you're going to fly 3D,if you're never going to fly 3D and just fly around a do a few loops and rolls etc. then standard servos will be fine.

Here is an ideal setup:

Aileron/Cyclic:Futaba S9202 coreless servos or S9252 digital
Collective:S9252
Throttle:S3001 or 9001 coreless
Tail rotor:S9253 or S9254(the newer one) used in conjunction with a GY401(very good gyro).

Don't use standard servos that don't have bearings on the output shaft as the vibrations on heli can damage the servo leading to which may cause a crash.One such servo is the S3004 that's why I recommend the S3001 if you're going to use standard servos.

Hope this helps!