Basic forecasting and scenario comparisons

Hello,

I've recently finished my education, and need to make some plans for paying
back a large student loan debt. I would like to be able to compare
hypothetical scenarios to see which scenario is more economical. I have a
basic understanding of Excel, but I don't have any formal training in Excel
or accounting and I could use some help.

One scenario I'd like to run is living alone versus living with a roommate.
I need to know how much faster I can pay down the loan, and more importantly,
how much money I will save in interest by accelerating the payments.
Likewise, I would like to compare the total mortgage costs of buying a one
bedroom condo and living alone versus buying a 2+ bedroom and living with a
roommate(s).

How difficult is it to customize Excel to do this kind of number crunching?
Are there 3rd party freeware applications available?

Any suggestions or thoughts are greatly appreciated.

Thanks,
Nemo