PSU blocks me for getting RX 570, it would be risky
Not sure if should I buy new or used like 3-4 months?Just buy the GTX 1050 Ti, if you can't afford a new PSU. For RX 570 AMD recommends a minimum 450 Watts PSU. Even though your current HP PSU isn't the very best, it should work with the GTX 1050 Ti.
Not sure if should I buy new or used like 3-4 months?
Im asking for GPUAre you asking this for the GPU, or the PSU ? I won't recommend to buy any used second hand PC component anywyas. Get a new one. At least you will have some peace of mind.