If any of your old coats fit, they will still work if navy, green or black. But you will want a new tech fabric if you can afford it. There is a trend towards shorter more jumper styled coats, but these look ridiculous on me since I need a tall size anyway, and so don’t feel you have to go with this trend. The classic shapes are still around.
I’ve found some Essex talent yarn shirts for cheap by getting past season patterns (on the inside of cuff and collar—who cares??).
There are are so many breech brands in the show ring now, with slight variations on the beige color. It doesn’t really matter if you get $300 breeches or a sale pair of Ariats if they fit you. Depending on your body type, I’d suggest looking for mid rise options and not low rise. Some of the fancier breeches will have more technical fabric. Might mean more comfort or might mean they are easier to keep clean than the old mostly cotton ones. But there are still some tech fabrics at lower price points too.
For helmet, get a brand that fits your head. 