What your board needs before installing anything.
Every product page states its own minimum versions next to its download, and those are the ones that count. When an add-on needs more than the baseline, it says so before you buy.
Admin CP, Tools, PHP info tells you your PHP version. Your XenForo version is at the bottom of every Admin CP page.
Yes, and several are deliberately built to. Levels feed on activity from the other add-ons, teams show their members' badges, and the trading cards can reward what happens elsewhere. Nothing breaks if you run only one.
In general yes. XenForo add-ons that follow the platform's own extension points coexist without trouble, and mine do. If you hit a genuine conflict, tell me which add-on and I will look at it. Conflicts get fixed, not blamed.
Everything is built against XenForo's own style properties rather than hardcoded colours, so a third-party theme, a dark mode or a style variation is handled without a patch. If something looks wrong in your theme, that is a bug and I want to know.
Premium XenForo add-ons, built, sold and supported by the developer himself. One-time payments, 12 months of updates, forums that answer back.