First home..found a crack...what could it be...please HELP ME!!!?

Buying my first home, saw a crack at the front door entrance on the concrete step slab. The home is 30 to 50 years old, and is a 2 story home. Is this crack due to the age of the home, or is there a serious foundation problem. The seller is asking for a resonable selling price, however he is wanting to sell as is. The seller stated his discloser was so limited because he didn't live in the home, he rented.If there is a problem this will cause me no room to borrow extra money just in case this may be a big problem.What are the cost?Should I walk away? Can it be fixed? What are the signs of foudation problems. First home, and im really scared. Please help.