when I taught quilt-in-a-day classes, I always had to troubleshoot plastic cheap machines- they just had lots of problems. Now I have about 50 vintage machines that are in good working order, a Babylock Ellisimo gold for embroidery, and a huge Bernina 820. Some of the things I look for(and will be keeping as I thin my herd) are: metal gears and cams, less vibration, easy to oil/clean/detangle, good size bobbin(the Bernina is awesome, the 201/221 are not), and stitch quality. If I need to hem a pair of jeans or resew my purse handle, I go straight to a vintage machine. If I am doing assembly line chain-piecing, I want speed and an easy to see fabric guide/foot edge.
I have found it better to buy the top of the line machine from a prior generation(used but refurbished) than a bottom of the line new machine.