How much work would be required to repair this roof?
Anything is repairable if you throw enough money at it. If you already owned this house having it repaired would be a consideration. A perspective buyer should have the current owner address this or the buyer should get an extreme discount.

A $100 inspection would provide you with a list, containing deserved discounts; the cost to replace bad wiring, plumbing; anything out of order. It's a bargaining chip, where the owner will probably fix what they can on the cheap or eventually agree to a reduced price. 50k(US) is a very rough ballpark for a roof gut, plus.