Once house is paid off can I take a new mortgage for appraised value?

Hi, my wife and I were thinking about selling our home.....which we would make around $10,000 after fees. We are thinking about buying her grandma's house for $160,000. If we paid this off in 5 years could we take out a new mortgage and for how much? We would like to use the existing structure and add on to make it double the size (ranch style). Grandma's house was a ranch, modular from the 60's so we most likely couldn't build up as I don't think it would support it. Thanks for any info!