A traditional old Singer will work for denim or medium ballistic nylon, and perhaps it could do a simple two-layer seam in very light leather. For heavier leather or multiple layers of thick material, a heavier duty machine will work better. Note, old industrial sewing machines usually aren't good for this--though they are sturdier, they operate at blinding machine speeds, typically with an on/off clutch that requires considerable skill to master. A slower sailmaker's machine or leather-purposed machine is better if you want to sew thick materials. (Although you can slow down the old industrial machines with a modern variable speed motor, that cuts back on the power available, which sorta defeats the purpose)

Note that the 600 series Singer and newer (from the mid 60's on) uses some plastic components in the drive train, not as good for heavy materials as the old Model 66 or 15's or the 300/400/500 series. I've made several amplifier covers with medium weight vinyl on my Model 66 and 503a machines.

One key is to use the proper needle for your material--leather needles and vinyl needles have different tips than general-purpose needles. Also set your thread tension properly for the thicker seams. I've found the old machines often have their thread tension mechanisms gummed up from old solidified lubricants, so they won't work properly until that little mechanism is properly (thoroughly) cleaned and oiled.