Since you only want a straight stitch machine, you should go for a vintage Singer 15-91 (or Singer 15-90).
They are easy to repair and clean all by yourself. There are lots of tutorials that can be found on you tube on cleaning and maintaining the machine.
Price is cheap. You can find these machines off CLs for under $100 and in a cabinet.
The harp space is large for working a queen or king size quilt.
The feed dogs can be dropped for free motion quilting.
The machine is all metal; so, it will last a life time or two or three. No plastic parts that will break on you.
And best part, the money left over from purchasing a Singer off CLs is that you will have money left over from your $600 allowance to purchase fabrics for quilts and/or purchase a FW to take to quilting classes.