Personally, I would buy a new machine. The old one can be fixed up as a backup.
When shopping for a machine, the very first thing you need to do is decide on a price range. If you are limited to $300 or less, then I would check out machine dealerships near you. Often a good used machine will give you more bang for your buck in terms of quality of machine. Deciding on a price range *first* will narrow down the choices to a manageable size.
In my opinion, you ***really*** want a needle-down option and at least a standard harp size (area underneath the arm where you need to squish a quilt for machine quilting). Judging by your post, you also want either a sensitive foot controller or a speed control on the machine. Another ***must*** on my list is the ability to adjust stitch length and stitch width yourself; do not get a machine with only pre-set stitches, as it is too limiting. Stick to brands such as Janome, Pfaff, Bernina, Babylock, Brother. (I personally would skip Elna, Bernette, Singer.) An advantage of going to a dealership is that you can sit down with your own fabrics and try out the machines before purchasing. (Do not use only the dealership's fabrics; they are different than quilting fabrics and do not always show up tension problems.) Be sure you understand the dealership's return policy before buying. I don't know about used machines, but usually dealerships will *not* take a return on a new machine unless you upgrade.
If you can't find something you like used at a dealership, then many people are happy with basic machines sold at Walmart. Just be aware that sometimes the harp size is small, and typically these machines do not have quite as strong a motor for machine quilting. Also be sure to understand the return policy if you try the machine and are not satisfied.