Looking for a mortgage?
While the seller of a property might be willing to do a 'vender take-back mortgage', there is no way of knowing this in advance. Therefore, you always need to go through the process of 'pre-approval' with a bank or other mortgage lender. It's a good process regardless: Your pre-approval confirms your credit rating; it also confirms the kind of interest rate you might expect to pay; and finally, it confirms how much you can afford to spend!
As for the differences between a bank or a credit union or a finance company, it mostly comes down to the interest rate you will get, the services that are included and whether you will qualify with the lender for a mortgage. It's worth checking out both big and small lenders in your hunt for a mortgage. It's also worth being pre-approved by more than one type of lender, in order to compare the kinds of services and interest rates you could get. Even a difference of percent could save you hundreds if not thousands over the life of a mortgage.
Why else get pre-approved? Well, probably the best reason is that a pre-approved mortgage helps you to set your house-hunting budget. Your lender will consider your credit history, income and payment history (as well as other factors) and then tell you how big a mortgage they will be willing to approve. Add to that number the amount of down payment you have and now you know your price range!
Alternatively, you might look at the amount that you've been pre-approved for and figure out how big a monthly payment that is. This might even give you a better idea; not everyone wants to have as much as 40% of his or her take-home pay tied up in mortgage payment.
So, once you know how much you can qualify for, you might want to take the time to work out payments, and then decide if that works for you. If not, consider lower mortgage amounts until you get a payment amount you are comfortable with. That lower mortgage amount becomes your key information in house hunting.