It depends on what type of sewing you want to do and what you think you'd get the most use from. If you're wanting to make your own patterns a set of curves is useful. I use mine a lot but never use a quilters ruler as I rarely quilt. For the same reason I don't use a rotary cutter as shears work better for me for the type of sewing I do. Skirt and bodice blocks you can draft yourself. Have a look at your library for pattern drafting books. Winifred Aldrichs books are good. She does a children's one as well. My blocks are drafts from this. If you draft your own you will need curves as it's a lot easier than doing it freehand. At least for me - I seem to incapable of drawing anything remotely curvey freehand. Helen Joseph Armstrong is also good but a bit more advanced. It would be worth doing your own as it's pretty easy and it's customised to your measurements. And saves quite a bit to buy fabric with!
And definitely look in DIY shops for a metre ruler. Mine cost about 5 quid from B and Q