Well I race a Tonykart EVR with a junior Rotax engine. However in the UK our circuits are relatively low grip so the softness of the OTK chassis family (Tonykart, Alonso, Kosmic, ect) suits us. However if you are just starting then the chassis make will do sod all difference, you are where the improvement will come.
However with engines its not which engine is best, its which is raced near you. Go down your local track, see whats most popular there and then go buy one of them. Then you won't be billy no mates at the track and you know the equiptment won't be holding you back.