As for the western aspect (I am english lacking), western attire usually will require:
Slacks or Jeans (Black, tan and earth tone slacks seem to be common, but western cut ones).
Western dress up shirt, usually button up front (in some events, more sparkle and glitter, the better).
Cowboy hat (good rule of thumb, match hat with pants or boots)
This is good for western pleasure. As for any Halter shows in the western world, same attire, but not so much sparkle, and a dress coat usually. You want to dress as closely to the attire the judges are in.
In reining and cutting events, you don't have to be as colorful in attire, but chaps are common with fringe.
Of course don't forget the belt and accessories. Western attire, if you want to bargain shop, I found everything for around the $300 range. Now, this isnt brand name all the way through, but good quality stuff with current trend style and coloring.
As for Tack, you don't have to get expensive. Saddles are about the only killing area. Say show halters, I will go through ebay
for. Reason being, most have a matching lead at no additional cost. And they are not always the best quality, but great for that show here and there, and eventually if they break, your not out alot. My problem is I match tack to the horse, and I start out in the weanling class and continue to show as the horse gets older. So I am not going to spend a fortune on a halter that is used once, especially when in 6 months I have to buy a bigger one!
When it comes to barrels and pole bending, a good used barrel saddle works great for both events. You can usually come across a good show type Billy Cook barrel saddle for a decent price. You can also purchase some "bling" for your saddle and attach it, oil it up, and you have a nice sharp looking saddle!
Good luck with the english stuff.. LOL!