How useable is this

Assuming everything works, it should be a very usable camera. Its only
serious drawback that I know of, apart from the fact that it's a
mechanically complex camera for which parts are no longer available,  is the
fact that the viewfinder is in vertical orientation and can't be changed to
horizontal. So to see anything horizontally the whole camera has to be
rotated 90 degrees. Of course if you photograph primarily in vertical format
this isn't a problem or if you can envision what something would look like
horizontally when seeing it vertically you also would have no problem (the
back rotates to horizontal so you can make horizontal photographs, it's just
that you can't see the scene in horizontal orientation without rotating the
entire camera).

I don't offhand know of a downloadable instruction manual for the Tech 70. I
doubt that the Linhof web site has one. It's a singularly useless web site
as is the web site of H.P. Marketing (the U.S. distributor for Linhof). If
you can't find one on line you might try John Craig. Doing a Google search
should also provide you with  information about the camera. IIRC Roger Hicks
wrote a short article about the Tech 70 in Shutterbug magazine about two
years ago and it may be available on the magazine's web site.You have to be
a little careful when reading about the camera on the web because Linhof
also produced a camera called the "Press 70," which is sometimes confused
with the Technika 70.  The Press 70 was a very different camera despite the
similarity of names.