Accounting firm seeks payroll processing software

For my accounting firm (external accounting services), I am looking for payroll and salary processing software. My responsibilities include, among other things, payroll accounting - data entry based on supporting documents, posting of routine business transactions, account assignment for documents, issuing posting instructions, processing current wages and salaries, and filing income tax returns...

Last but not least, the software should meet the following requirements:

• Payroll processing
• Hourly wage recording
• Overtime calculations
• Optional processing of various time models
• One-time payments and bonuses
• Part-time or multiple employment
• ELStAM retrieval
• Income tax returns
• Income tax certificates
• Company pension plans
• Social security reports
• Garnishments and loans
• DATEV interface
• ELSTER interface
• Multi-client capable

A single software workstation is planned. The system will run on an MS Windows operating system.