My company has a stand-alone web-application external to Salesforce that provides SaaS tools for financial advisors. We would like to make our software capabilities available to financial-advisors from within their Salesforce accounts. Desired integration points include:

 * Allow Salesforce users to log-in to our software automatically from Salesforce - how would we consolidate the accounts? 
 * Allow advisors to run our SaaS tools against their leads (i.e. client profiles) that are held in Salesforce.
 * Embed our Web-UI within Salesforce - we have multiple tabs on our site. Is it possible to embed some of these in iframes on a user's Salesforce console? 
 * Make our software tools available as Salesforce applications - is there some kind of store for plugins? 
 * Manage paid-subscriptions for use of our software

Do I need to rewrite our web-application as a Salesforce app in order to integrate? 

I've not had much luck finding the right integration documentation or examples for what I'm trying to achieve. Hoping someone can point me in the right direction! 

Thanks,

Alex