Never hurts to get the thermometer, you'll use for a multitude of things if you make candies, caramel sauce, and pasturization in canning.
If you go without a thermometer, pay close attention to bubble formation (small bubbles, followed by big bubbles, followed by small again) then to the change in color--when it gets just a little caramel colored, it's done. Stop the heat right away. Work fast, add whatever it needs, and pour it out quickly.
My favorite toffee is with almonds added (too) and then poured into a greased sheet, topped with chocolate, spread thin, then break it up into pieces.
Once you've done it a couple of times, you'll be a master at it.
Tip: don't cook candy with a rubber spatula--had a friend try that, it's not a good idea. Rubber spatulas melt.