Hire the agency to provide a team. Request that the team work onsite. After a while, offer people to jump ship to your company. If there is a clause in the contract preventing poaching, consider that financial penalty a commission.

Happened to an agency I once worked at. We sent 6 people (4 were full time, 2 were contract) to a client to work on their project. After a year, none of the 6 people were still with the agency. The contractors were let go after the contract expiration, 3 of the 4 full timers took jobs with the client, and I left for another company.

I'm pretty sure if you went to a digital agency and wanted to hire 5 people 40 hours a week and pay the rate they would work for you exclusively until the point you no longer want them to.

Better off is software consultancy groups. They're more used to this set up.

There aren't many permanent arrangements in software.

Ah yeah, in that context it wouldnt be a digital agency.