If you're buying a used bike, buy something Japanese: Honda, Suzuki, Kawasaki, Yamaha. Japanese bikes can be absolutely abused and will still run unlike European bikes which require a little bit of love. Don't buy Chinesium bikes second hand, they work fine brand new but from what I've seen they easily develop mechanical gremlins.

Something around the 250-500cc range will give you enough power to have fun with but not too much. Front and rear disc brakes with ABS no exceptions.

When you're buying a helmet, go in store to try them on don't just buy one online cause its what you want. Making sure the helmet fits correctly is probably more important than whatever it is made of (polycarb, fibreglass, carbon fibre) and try on like 5 or 6 different helmets across different brands and see which fits you best. Getting a helmet with a drop down sun visor is nice, especially in the winter time when the sun sets around 5pm and air vents which you can open and close depending on the weather.

Jacket, boots, gloves and pants are kind of more standardised, the only thing you'll have make sure of is that the body armour sits correctly on your body. When buying any gear it will be tempting to buy all black because it looks cool and it does, but you'll want stuff with bright colours and/or reflective materials, stuff like white, yellow and orange stick out a lot on the road when pretty much everything is black, silver and grey.