I never bothered with any kind of special cycle clothing for maybe my first 25 years of cycling. Finally I took the plunge and bought special cycling shoes for clipless pedals having had toeclips for decades. Worthwhile, but then you do need bike shoes just for a casual ride, albeit mine are fine to walk in. Other stuff; whilst not essential by any means cycle specific clothes are slighly more comfortable and convenient - eg thin so not overwarm but slightly windproof jackets with pockets at the back. These are quite good compared to a jumper or whatever. A winter fleecy windproof cycle jacket is brilliant in bad weather and far more use than a true wstaerproof. Whilst I do have cycle shorts I was quite comfortable in ordinary trousers, but bib 3/4 lengths are good for comfort, pariculalrly so if raining, and for not getting oil on your trousers. Gloves - well you need gloves in the cold weather regardless but cycle ones have extra hand padding which is a good thing. All in all, none are essential but all have benefit and I guess it's not wholly additional cost as your normal clothes last slighly longer if you're now not wearing them cycling.
Helmet is of course a controversial subject, but for what it's worth I used to wear one, an "early adopter" even, but I no longer do so having looked into it a bit.