I'd say spend a bit more on a better one because if you sew regularly you will very quickly finding yourself using the over locker for almost everything.

I've got a brother 3034d and love it but the Janome that Astrid was talking about is great too.

Rolled hem is useful, 4 threads is essential (I often just sew things together with the over locker and 4 thread gives a better result) and being able to disengage the cutting blade is great too.