Zendesk Sell is Zendesk's standalone sales CRM: leads, deals, pipeline, contact management. It's a genuinely separate product from Zendesk Support with its own plans and its own pricing, not a feature bundled into Suite. It does integrate with Support so sales and support can share customer context, but buying Support doesn't get you Sell.
Sunshine Conversations is a different thing entirely: a messaging API, originally built by a company called Smooch that Zendesk acquired, that unifies conversations, users, and messages across more than a dozen channels (WhatsApp, Facebook Messenger, SMS, custom in-app chat) behind a single API. It's what you'd use to build a consistent conversational experience that doesn't care which channel the customer actually messaged you from.
The original "Sunshine" platform, an open customer-data platform with custom objects built on AWS, is a separate, older piece of the same branding, and it's legacy: Zendesk has been retiring the standalone Sunshine custom-objects platform in phases, folding similar custom-object capability into core Support instead. If you see "Sunshine" referenced in older material, it's worth checking whether it means the current Sunshine Conversations API or the legacy platform, because they're not the same thing anymore.