Hello,
This is an expert developer and I would like to estimate your request.
First of all, which kind of application do you want to be built? You said it has to be downloadable, do you want a desktop cross-platform solution? Something that could also run on iPhone and iPads?
Could you detail the accounting type do you want to perform? Which kind of reports do you want to extract from it?
Looking forward to your reply in order to go on with the estimation of your project.
Best Regards,
David