Need a plugin for accountants website

Some of those tools are pretty specialised. There are plenty of calculator plugins here you could probably find a pretty good mortgage calculator there that suits your design/requirements and then once you’ve got it installed you could use it as a model for creating your own plugins with the specific functionality that you want by replacing the functions with the ones on the existing site.
Hope this gets you a bit further on.
Give us a shout if you need any more help!

Martin