I personally would never buy anything but brand new.
Find a shop or phone if you don't have a local one. Tell them what you hope to sew and ask their advice. Good luck
Don't listen to
@Lowena - she won't have
anything that's not brand spanking new!
If you ask a dealer for something new in your price range they will only be able to offer cheap, plasticy, lightweight machines that won't handle several thicknesses of denim.
The vintage Singers mentioned are excellent workhorses, especially the 201 - but they are straight stitch only. Mine made piped box cushions from upholstery fabric which is thicker than most denim without complaining and at times it had to go through six layers of fabric.
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However, if you want to sew garments where you need to neaten the seam allowances something with a swing needle to do zig-zag stitch would be better such as a Singer 306.