produce a interactive mortgage program, preferably on goggle doc or #2 excel

I'm looking for a freelancer to help me produce a interactive mortgage program. The features that I would like the program to have include calculating monthly payments, total interest, amount paid per month with dates ( payments will vary ) and remaining balance due. Also two columns that will be used for manual entries. Entrees will consist of numbers, symbols and letters. Google docs would be a preferred way to do it . I could make excel work. Time span would be 30 years. I do not require data validation for accurate input, though I would like the program to be user-friendly for non-technical users.