Depends if you like spoilers. The movies are not super faithful to the books but close enough to ruin the main plot points. That said, I read the game of thrones books first and still loves the series before it went beyond the books and soured. 

The first book is the classic. I read all six written by Herbert as a teenager. They are all good but but get weirder and weirder. 

And is the 80s movie by David Lynch worth watching? Or would it just look silly and campy (or at least dated) now?

Yes the movie is silly, campy and a bit dated but still worth watching. I feel like it gets a lot of unwarranted flak. 

Regarding the later books they do show what the whole point of it was so are worth reading but get a bit crapper after Chapterhouse when his son takes over.

I re-read the book recently too, and it always leaves me wanting more, but when I finally read Dune Messiah a few years ago I recall being disappointed with it.  Obviously its a subjective thing though so you should deffo give it a go and see what you think.

I like the 80's movie though its very much showing its age now.  Still has some great scenes.    

Dune Messiah feels like a bit of a filler to set up for Children of Dune which is where it gets rolling towards the point and gets weirder.