Your monthly payment should include three components:
1) the mortgage payment, which is based on the principal, the term, and the interest rate.
2) the mortgage insurance, which will depend on rates in your area, and
3) the real estate tax amount, which also depends on rates in your area.
The first you can calculate in any spreadsheet or financial calculator. The other two are unknowable to those who don’t know where the house is.
As to what you can afford—that depends on your current debts (how much you carry on your credit cards), your income and your expenses (of which debts are a part). As a rule of thumb, I’ve heard people say your spending for your monthly housing payments should not be more than one-third of your monthly after-tax income. Others say it can be a bit higher. It kind of depends on your income and your other expenses.
What kind of car do you drive? How many cars do you have? What are the payments on the cars? Do you spend a lot on food? Clothes? Entertainment? Vacations? Etc, etc?
My suggestion is that you be conservative about your spending. Also be conservative about what you can afford.
It is unusual, in this day and age, to finance one hundred percent of a house. Your relative may think he’s doing you a favor, but I think the traditional wisdom of putting down 20% on the house is a good idea. It protects you from market fluctuations. Maybe you are buying a house that has already seen a big drop in value, but that’s no protection. You don’t know if the economy could get worse (as it surely will if Romney is elected) and you could quickly find yourself upside down on the house (the mortgage costs more than the house is worth).
So I would be very cautious about your mortgage. Draw up a budget. Put all your expenses in. Then increase them by 20%. If you can afford the house under those conditions, I would feel safer about buying it. But if that would be a problem, then I would back out. Also, you have to think about how secure your income is. Could you or your husband lose your jobs and still be able to pay your mortgage? Or could one of you lose a source of income? If not, how stable are your jobs?
You are nervous and rightly so. But there are answers to your questions. You just have to do the budget. Frankly, I’m a little worried if you haven’t done a budget already. This shows you are unprepared. Usually people do this when they first start thinking about buying a house, not much later on when they are about to close.
So do your homework, as @CWOTUS just said, seconds before I submitted this.