You can do lots with a normal sewing machine so you don't need an overlocker, but as you get more into it, you realise they speed things up and do knit fabrics much better.
A lot of the clothes I want are made from jersey fabric which is stretchy, and while it can be done on a normal machine with a zig zag , the overlocker stitch is stretchier and faster. While it isn't that time consuming to straight stitch and neaten the edges with zig zag, I am sure the more dressmaking you do, the more you will hanker after an overlocker. I totally intended for my Bernina 1008 to be my one and only machine for everything, but less than a year later I now own an overlocker.
If it was a choice on where to spend your money, a cheaper second hand sewing machine and a higher quality new overlocker would be my preference.