Dear Sarabas,
Yes, it's clear what you're after in terms of appearance and interaction. And, fortunately, today there are tools and frameworks that allow a high level of interaction to be created with less effort than in earlier times.
For example, there are frameworks that enable what's called "data binding". What this means is that you can "bind" the results of one field to another. So that when one field changes, the others do as well - automatically. It's possible to set things up in a way that all parameters in a mortgage calculation - rate, terms, etc. - are free to be manipulated to see how they all interact. Point is that today, with the JavaScript tools available - and if you know how to use them - you can build a highly engaging, dynamic page, that'll show nicely on everything from a Mac Cinema display to a 5" smartphone, all while allowing ease of use.
Clearly, accuracy is most important here. Generating dynamic graphs based on the data is also quite easily achieved; again, there are some terrific JavaScript libraries available for charting, easily integrated into the overall application.
I've built mortgage calculators for another user on this service years ago, so I am very familiar with the math underlying them. And we have many more opportunities today to add more value to users. For example, we can create scenarios - compare one set of figures to another - and the user can save/print them. And much more...
Looks like a fun project, and I'd love to help!