I just bought a house circa 1753. Of course, now that I've paid for it, I find the termite damage. There are two spots of concern. One of the beams in the basement was eaten about 1-2 feet in from the wall. The other I can only see in the floor boards above the foundation, but I fear they ate the sill on their way or even worse, they may have gotten the corner post.

What I wonder is: what are the techniques to repair these three components (beam, sill, and post)? Is it something that can be done by me or my wallet ?

It also appears that other parts of the sill have already been replaced but some of the beams no longer rest on the sill, but instead upon the foundation. Should I be concerned about that?

The first step you should take is to solve the original problem. You need to make sure to get rid of the termites or confirm they are no longer active (if they were treated before your purchase).

Second would be to hire an experienced timber frame repair person to survey the extent of damage and recommend repairs. If the house is relatively intact, it would be worth paying the extra cost of doing the repair without removing historic fabric. This work requires a specialist.

There are several Guild members listed in the TTRAG membership list that might help you find the right person. Once you have found a potential conservation specialist, don't hesitate to ask for references and permission to contact previous clients.

Good luck with a great goal!