That Rooster Booster is fine.
There are certain circumstances where you may need more Vitamin E than what something like that will provide. Say, you have Wry Neck, then you would ramp up the E and give B-Complex. I would not buy Selenium at all, it can be toxic and only a very small amount is ever needed -egg, tuna, sunflower seeds provides sufficient amounts.
A lot depends on where you live an how easy you can get products too. I DO keep a few things on hand - poultry vitamins of course and first aid supplies. Syringes, I order those online - Ebay and Amazon are great. Needles I can order those or pick some up at TSC.
If I needed something special like B-Complex or E, then I would just send someone to the store - in my town grocery, pharmacies and Walmart are open 24/7.
It doesn't hurt to have a good "all purpose" antibiotic on hand. Baytril would be a good one to choose. I do keep something just in case too.