In order to get a mortgage in Belgium you must be under 65 years old and submit proof of income (copies of your last six months' payslips, personal bank statements, etc.). You must also prove that all your existing liabilities (rent, loan repayments, etc.), plus your mortgage payments, won't exceed 35% of your net monthly income. The lending will be based on your ability to pay, and won't take into account any rental income from the property.
You may also be required to obtain a survey of the value of the property. Surveys are done by an expert valuer approved by the mortgage company. The value assessment is often based on the replacement or forced sale value, not the market value. You will have to pay the expert valuer´s fees. Note that this survey doesn't always give you signs about potential problems with the building.
Mortgages have typically a 20 year duration. The minimum loan is of €30,000, with a maximum loan-to-value ratio of 85%. Since 2008, the market has been dominated by loans with interest rates fixed for the duration of the loan, or at least for ten years.