We need a CRM with four important modules which are: 

1. Quotes 
2. Contracts 
3. Invoices 
4. Support

 We would also need a module for Contacts, Leads, Products, etc. But the most important are the first 4. 

The Quotes module should allow us to input all the products the customer wants and then have a formula so that it calculates a leasing monthly payment depending on the initial deposit. Then when a quote is accepted we need to be able to convert it to a contract and then start billing him each month for the amount that the contract stipulates which was calculated on the quote. Then for support it would be only cases and solutions with a knowledge base for our technical support team. 

We would also need to be able to specify which users see the support modules and which ones wee the sales modules and which one sees everything.