The best trend to follow in the hunter ring is going to be the "traditional trend" haha. Traditional is always the best way to go.
The helmet is going to be entirely up to you, first and foremost is needs to fit you properly. Unfortunately the popular helmets are the pricey ones like Charles Owen and GPA. CO makes a helmet that is cheaper than the ones that you posted: Charles Owen JR8 Junior Helmet - ASTM/SEI Certified Helmets from SmartPak Equine
, but again, this is all up to you. Any of those helmets will work.
I wouldn't worry about the belt. I have seen so many belts in the show ring from plain leather ones to blingy ones. Your best bet is to stick to something subtle and one that fits with the belt loops on your breeches (skinny belts tend to look funny on the newer style breeches with the wide waistbands).
Those gloves are perfectly fine in the hunter ring, just make sure they're black.
Breeches - for the show ring, stay away from pull on. The side zip/front zip is a preference thing. Some brands of breeches fit me better in front zip while another brand fits better in side zip. Try on a few different brands to see what works with your body. I have recently been very happy with Equine Coture's breeches and they are reasonably priced...I use them for schooling but they would work just fine in the show ring.
That jacket is good, just make sure it fits you correctly. Black is usually seen more in the jumper and Eq ring, but has become less out of place in the hunter ring. And the trend now is the soft-shell jackets with stretch, like this one.
Don't forget a show shirt...the ones with the wrap/snap collars are currently in (and such a brilliant idea, no more lost collars), but one with a collar works just as well.
Well fitted tall boots will also be very important. If you are going to splurge on anything, these should be the item (except a helmet, make sure you have a good helmet too). Spend the money upfront and take care of them and they will last you a long time.
For the jumper ring, you can use any color pad you want, but you can never go wrong with a plain white square pad. Make sure you have a fitted hunter pad for the hunter ring.