One item that I didn't buy until about year three or four (but wished I had bought earlier) was a TOWA gauge to measure the tension on bobbins. Once I had this - no more problems with tension on the back.
I would also make sure to have at least one extra bobbin case on hand; there will inevitably come a time when you drop your bobbin case and it will have the slightest little bend or dent to it and you'll find yourself needing that replacement. Some manufacturers or dealers actually have kits of spare parts - ask to see if your machine will come with one for free or a small cost.