Robust Security for Freelance Platform

I’m preparing to launch a brand-new freelancing marketplace and I want its security to be rock solid from day one. The platform will process personal details, handle payments, and store highly sensitive client files, so every layer—the data, the users, and the network itself—has to be locked down... (Budget: $10 - $100000 USD, Jobs: Cloud Security, Computer Security, Incident Response, Internet Security, Network Security, Security, Testing / QA, Web Security)