AIM-listed K3 has raided FDS' cupboard twice before, buying its Microsoft Dynamics NAV unit Clarita for £1.1m in April, and its Sage 200 business unit for £1.25m at the end of 2010.
Along with repayment of FDS loans totalling £410,000, a further cash consideration of £400,000 will be payable pending completion, and a cash earn-out of £1m may become payable next year based on performance, said K3.
Cardiff-based FDS boasts over 300 customers in the UK, with names such as TV producer Ideal Shopping Direct and security and fire safety specialist Chubb on its books.
It has also developed its own e-commerce solution, called Gravit-e, which integrates with Sage and Microsoft products.
K3 claimed the acquisition would make it one of the largest Sage partners in the UK, with annualised sales of close to £8m.
"The deal represents further progress in our strategy to acquire customer bases with strong recurring revenues, bringing over 300 customers where we see excellent opportunities to sell additional software services and solutions, including hosting," said K3 CEO Andy Makeham.