It isn't a silly question at all. The square filters are convenient to use, but can be bulky to store and carry around not to mention the price.
I have acquired over the last few years two screw in filters, a ten stop ND filter and a circular polariser. Both are Formatt-Hitech Firecrest and fantastic quality. I paid probably half rrp for each as one was on sale and the other bought used and have never really felt the need to buy a filter set to significantly up my landscape game so to speak, particularly as grads etc can be applied in LR so easily.
My advice? Get a polariser and ND filter (perhaps a six stopper to begin? The ten can be too long sometimes, particularly at sunrise or sunset when there's less light anyway) in the biggest thread size you need to cover your largest lens IYSWIM, then step up rings to fit this to your smaller lenses.
The step up rings are a couple of quid each and if you put a wanted in the classifieds I'll bet you'll get a decent couple of filters for a fraction of the new cost.
Happy hunting