The main considerations when buying a machine are, IMO:
* does it have a metal chassis? To get one you will have to go up the market. Cheapo machines have nylon chassis and they flex under any kind of load, meaning the needle won't penetrate, eg, a denim hem and its associated side seams.
* is it electronic and, if so, are replacement boards available/likely to remain available in the near future? If the electronics go and can't be replaced you will have at best a straight-stitch machine and at worst a door-stop.
* what kind of bobbin does it have? This is a matter of preference but I don't get on with bottom-front loading bobbins and will pay more for a drop-in bobbin.
* what stitches are you likely to use? Basic electro-mechanical swing-needle machines offer a variety of zig-zag stitches and either one-step or four-step buttonholes. Early C20th machines are generally straight-stitch only: I've heard stories of buttonhole attachments for these but never seen one in use myself.
* finally, exactly how good is its top stitching? This for me is a deal-breaker. If the machine doesn't do a pristine, straight-as-a-die top/edge stitch without a great deal of wrestling, just no. I have a Brother Super Ace II (now 25
years old) that does great buttonholes, stretch zig zag, etc, but its top stitching isn't up to snuff and it's a struggle to get edge-stitching neat. I believe it's something to do with the pair of feed dogs being wide apart. If I make a flat fell seam to the inside of a garment, meaning the bobbin thread is on the outside, it's a mess. For that I use a 1978 Singer and it's immaculate.
Once you've defined your broad requirements you can narrow down your options. It's easy to be dazzled by complexity but if you're never going to use all the bells and whistles, why bother?