Outsourcing recruitment for a Not for Profit organisation

I am heading up the internal recruitment function for a charity that runs care-homes across the UK Due to budgetary restrictions we are unable to increase the size of the in-house recruitment team to meet a sizeable recruitment need required following a positive culture-change at a number of sites We have currently approached a small advertising & recruitment firm, a medium sized virtual recruitment partner and a FTSE Corporate Recruiter/RPO I will be going through the proposals to assess which solution best fits the organization and measure the ROI - Has anyone got any guidelines on things to look out for, or the best way to objectively compare different solutions at different prices against our requirements? Also, has anyone had any experience outsourcing the resourcing side of talent-aquisition within my sector (Ideally for a PubSec/NFP, rather than a corporate) Thanks in Advance