Ask a local realtor. My guess is it would hurt your home's value, because it's not what most people are looking for. Most people want a formal dining room that seats maybe 8-12, not 30. And while most people like a family room, they may also want a living room.
Could you swap the two rooms but gain the storage without built-ins? Buy sideboards, etc, which are very cheap in thrift shops. That would make it easy to swap back if you put the house on the market.
Have you maximized the storage in your kitchen? That is where I would start.
Q