I'd do 2 things. First, the formula for a payment on a standard mortgage is

Loan Amount / a[n] where a[n] = (1 - (1/1+i)^n)/i

I'd set a variable v where v = 1/(1+i). Make every portion of the equation a variable so if you have to change things, 1 change makes all changes.

Secondly, Your Loan amount should be the inital price of the house minus the down payment. You could have a variable for initial loan and then down payment, number of months from the # of years input you have. Make interest rate a variable as well as months. See here:

Mortgage Payment Calculator
That will show you the entire math behind a standard monthly mortgage. It's written in javascript, but we could always convert that quickly to Maple. Let me know if you have questions.

If others besides yourself will be using this, you may want to account for user error on inputs, such as text and negative inputs since no input on a home loan should be negative. How is your interest rate coming down? Is 5.5% going to be entered like 5.5%, 5.5, or 0.055? In general, if others are using this, it's best to make it as simple as possible for them, and you handle the automation.

I'm not sure if Maple has code that respects Regex; If they do, that's something that would take care of most of the above in one quick move.