Definitely try lots of styles and makes on. Boot choice is very much down to the individual. Eg lots of people like Ariat, but I had to sell my Ariat Bromonts - couldn't get on with the little laced bit up the front. Whatever you do though, don't buy those Hunters or any kind of welly for riding. A relative broke a leg very badly when her welly got stuck in a stirrup as she fell off. Buy proper riding boots - there are loads of them out there! Have a look at jodhpur boots and chaps/gaiters. Personally I prefer long boots, with Mountain Rider being my make of choice if I have the money, but I have a pair of Toggi Calgarys atm which are very nice.
Jodphurs are sort of important, although not essential. I ride at home in jeans a lot of the time, only wearing jodhpurs for shows and hunting (and when I want to look tidy! I don't wear immaculate jodhpurs to pick up the kids from school so that I can go 'Look! I am horsey!' like a couple of the mothers seem to though
) You will get cold, and likely chafed, in leggings eventually, I reckon.
I ride in a gilet. They are EVERYWHERE this autumn. My current one came from Sainsburys, but I've had ones from horsey shops too. There is no discernable difference between the horsey and non horsey shop gilets. I put whatever I have to hand underneath it. I am not a very stylish rider
. Except (yes, you guessed it) when at shows or hunting.
If you are looking for a decent selection of what the fashionable horsey peeps are wearing atm, look at Robinsons and Rideaway online. Then have a look on ebay for the same but cheaper.