Help save our 300+ year old cottage

Having recently put all our savings into buying our dream home together (actually more my dream than my husbands...lol) we need help from some amazing people. When we bought our Grade Two Listed, 300+ year old Cottage a few months ago, people told us it would be a money pit. However, I like to look on the brighter side of life so I never quite believed them. I'm now having to eat my words as having already found a few problems, we are now faced with a major one. The cottage is built with a traditional oak timber frame, to which we have just discovered that some of the oak beams are rotten - to the point that my husband was able to put his finger all the way through one of them!. So we are now faced with expensive and extensive repairs to the oak frame to keep our historic cottage standing. I dread to think what lays underneath the plaster boards,between the external and internal walls. We do know for a fact that in certain areas, the rain water has penetrated and turned certain internal wooden support struts to powder!. I know that this is a purely selfish ask, but I'm not sure where else to turn. I would love to keep this cottage standing for another 300 years!! With your help, I'm hoping this can be achieved.