I would look for a machine with the largest harp you can find. Look at the height as well as the measurement to the right of the needle. the machine should either have a built in even feed, or an even feed foot (walking foot) made by the manufacturer. If you want to FMQ, you will also want a darning foot and the ability to drop your feed dogs.
After that you need to decide if you need any decorative stitches. You at least need a zig zag to piece batting.