Bookeeping software to develop

I am in the process of developing my accountancy website and one of the key features I would like to include is for clients to log in and update their invoices, expenses etc which allows them to do all their bookeeping for them.

I really like www.freeagent.com and the key features in this software.

Really need a professional developer who can first establish my needs my reviewing the above website and develop this software which I can use as an addon to my website.

Preferably someone who has done something similar or with some accounting background to understand what I need without too much hand holding.