Thanks, your knowledge really helped me out.
I don't really plan on buying a camera. But the reflector telescope seems to be a good option, as it can easily be used for thermal and visual observation of the sky.
Regrading auxiliary telescopic lenses, I saw this example
Boris claims to build the scopes on his own.
For a reflector telescope there are different designs. The Newtonian with an eyepiece to the side and a angled secondary mirror.
Or the Gregorian/Cassegrian way with having a big hole in the primary mirror.
For thermal use the biggest problem will be the eyepiece/ focusser as those aren't build out of LWIR transparent material, and even a small lens can get really expensive.
My current goal is to find some items arround me just to see if the effect works.
I tried different reflective rounded objects like transgender can lids, spoons, plates, bowls, follower pots.
I will try to visit a local store which has some real telescopes and ask them about testing it thermally by removing the eyepiece.