Start with (free). If you outgrow it, upgrade to BBC Core ($249) or subscribe to EastWest ComposerCloud+ for one month ($20) to try Opus properly.
Your computer’s health and your musical workflow are worth more than a risky crack. Opus Vst Free Download
If "Opus" refers to a specific VST plugin, it could be a virtual instrument or an effects processor. Without more details, it's hard to provide specific information. Start with (free)