Deadlift PR is 220 kgs, without a belt (but using straps). Recently bought a belt which has taken my working sets from struggling on 180x5 to comfortably doing 190x5. I only use belt and straps for my working set. I don't think it is necessary at all, but is nice to use at higher weights. I remember around 140kgs I thought about it, but improved my numbers by changing programs to PPL (which had more volume that indirectly helped deadlift, such as romanian deadlift etc). Keep in mind that there are studies showing that belts don't prevent injuries. The number you will pick is inherently going to be arbitrary, and it depends on how heavy you are, but I reckon they definitely aren't necessary until nothing else you do brings your deadlift numbers up.