I just got finished replacing the right side inner sill on my TR3. The picture that you showed makes yours look fairly good. What I can tell you is that, what was rusted through was bad. But the portions that were not rusted through were good. If I had yours to deal with I would simply replace metal where it is rusted through( it looks like your is only bad on the very front of the sill shown). And then I would treat the outer part with a paint that is used to bond over a rusty surface. I tried this stuff and it really works nice. If you are worried about the inside, I would try to clean it ( tip the body and use a pressure washer and let the crude flow out the opening) out while the front end is open and then treat the inside the same as the outside with some of the rust covering paint. It could be sprayed inside while the end is open. But I see no reason to open up any holes elsewhere if the sill is not rusted to the point of having holes through it. The sill I replaced was missing all of the top half of the front piece along the footwell and the entire front end. The drivers side was only missing part of the front end as yours appears to be.
Charley Fitch
