Hi!
For ordinary sport .40-.60 planes with a span of 140cm-180cm just any standard servo is suitable. A standard servo weigh around 40-50g and pulls 2,5-4kg (Meassured one centimeter from center) and can be analog or digital and have either brass bushings or one or two ball bearings.
Nylon gear is preferable to metall gear due to less play.
Futaba, Hitech, JR, Spektrum, Multiplex,Sanwa are safe/good brands! Dyamond and Blue Bird are others that work good too

Futaba 3003, 3001, 3004 , Hitec 422, 425, 225 . JR 508,511,4001, 801 Spektrum DS 821...are just a few examples of servos I have used during my 36 years in this hobby.