Need to pick up a system (preferably with a web interface), which includes:
1. The CRM module features:
— split the transaction into several stages (negotiation, signing contract, etc.)
— to attach documents (as implemented in sugarCRM, for example)
— base of counterparties
2. Billing module, which is able to remind you of periodic payments. For example, if you invoice the company for 10,000 rubles every 3 months, the system every 3 months to remind you to do it.