If it's with the Veteran Horse Society, and any veteran other classes I imagine, you turnout to type.
Put a picture of her up and then people will let you know what they think. If she's Show Hunter Pony type then a tweed jacket will be fine, if Show Pony type then you will need a Navy Jacket. The rest of your workers clothing should be fine. Although spurs aren't allowed under Veteran Society Rules. You could also have an age badge for her.
For her, brown tack is prefferable as it's more traditional as is a straight cut saddle, but it shouldn't be of too much influence, better the saddle fits you and her in my opinion.
Also, if a show pony type she'll need a pretty, blingy browband of your colour choice, and your decided amount of bling, and a fine raised noseband.
If Show Hunter Pony type then keep it plain, plain browband, plain noseband.
Bitting wise up to you, your preference. Judges seem to prefer to doubles, but obviously what you mare goes best in, is the most important factor.
In-Hand wise I'd just use your normal bridle and your riding outfit, if you can run in your boots!
If not, some trousers, that are a plain colour that contrasts with her leg colour and suitable running shoes.
Have Fun! I love the Veteran Showing Society, had a ball the last two years I've done it.