Management consultancy with accounting services seeks office solution

I have founded a management consultancy with accounting services and am looking for software that will help me handle my clients' office work. Initially, I will be working for clients in the construction industry, but expansion into other industries is conceivable.

I have the following requirements for the software:

• Multi-client capable, as I have between five and ten clients
• Quotation
• Processing of service specifications would be helpful
• Invoicing (including partial invoices)
• Dunning
• Importing and posting digital incoming invoices
• GoBD-compliant accounting
• Digitized accounting process
• DATEV interface to tax advisor
• Windows-compatible

One software workstation is required.