Guys thanks for all your helpful suggestions, I feel more confident about the right way to tackle this job.
Holzhacker, I'm gonna follow your advice and cut out a small portion of the jambs to slide the sill in. Your detailed explanation is very useful. Your other method looks quite complicated for me to realise with the tools I have, I have no planer for instance. Also, the difference in height between the front and the rear is quite big. Just one question remains for me, what is a "bead of butyl or urethane" exactly? Tried to Google it but to no avail.
Brice, thank you for your explanation of all the (rare) words. Indeed, as a foreigner all those trade specific words are difficult. And your visualisation with Sketch-up is also very useful. I didn't fully understand Holzhackers first method right away, making the sill out of 2 pieces and sliding a small piece under the other, but with that nice little picture of yours I now do. I think I'm gonna opt for the other method though, seems easier to me with the tools I have.
I'm not even close to being a pro. That said, the first thing I would do would be to check the condition of any surrounding wood. In the Pacific Northwest, where I am located, it would be hard to believe that only one piece would be that rotted without there being other, adjacent damage.
If there is other damage, you may have your answer.
Amazing as it is, the jambs are in pretty good condition. There is some rot, but it's really only very superficial.
You're probably constructing the new treshold ( onderdorpel ) in hardwood like Meranti or Merbau ?
Nope, it's gonna be some type of pine, Scandinavisch Grenen. Don't know what they call it in English. Unfortunately, the neighbour I have to do this for is a bit frugal and moans at every extra euro he has to throw at it.
But he's an old man already who expects not to live too long anymore, and as he says "as long as it serves my time, it's ok. And my time isn't long anymore". But Scandinavisch Grenen is pretty good wood actually. Just not as expensive as Meranti or Merbau.
Epoxies are OK, but I've more or less abandoned them completely in favor of Urethane-Acrylic compounds. You know your way in the painting world - google "Sikkens Componex Fast" for example. They're not sensitizing like Epoxies are, and perform better in my experience. The double "glueline" acts as a barrier, so the hardwood bottom part starts to act like one of these stone jamb-bottoms - don't know what they're called in English, but you probably know them as "neuten" .
Never heard of 'neuten' actually. I know what a 'neutje' is, but that's something completely different.
I would love to work with Sikkens Componex WR Fast, as I'm a big fan of everything Sikkens makes and with paint it's the first brand I go to, but this stuff is just not in my budget. I also feel I really don't need it. I actually always use a 2 component woodrot filler epoxy from the Hema. I know, that's a surprise, the Hema, since most of their paint related stuff is not what a sensible man would buy, but this one product is really satisfactory for me, and as it goes for 2 component fillers, dirt cheap. Been using it for over 10 years now, with no problems on the long run. It works really good when you apply it, great adherence, and after it has dried, it is very strong, easy to sand and model, and still has some flexibility in it so it will move with the wood.
From your pictures it's hard to determine if there's a groove ( waterhol ) in the bottom of the profile ( druiplijst / lekdorpel ) that is mounted to the door. You might want to look at that as well.
That profile/lekdorpel you mention isn't in the best shape anymore, but as for now, I'll just leave it alone. The guy doesn't want me to make everything in top condition, he just wants me to prevent it from falling apart within the next 5 years.
But thanks Job, for adding a lot of useful information.
Once again guys, thanks for your input. It will help me very good.