If all you want to do is straight stitch (no zigzag, no fancy stitches) and there is a working Singer 301A among your used advertisements, snap it up. It's a workhorse -- easy to thread, easy to maintain, manual and parts readily available, modern attachments such as a walking foot can be found to fit it .....
If you don't feel confident about assessing machines, do you have any sewing friends that can go with you? Take some pieces of fabric to try out.
I have a Pfaff 1540 and another old Singer. Neither holds a candle to the Singer 301A in terms of usability. The Pfaff has integrated dual feed, which is the bomb for certain tasks, but you pay for that in tension fussiness, complex threading pattern, etc.