The last price I saw for Achim Engel's plans was $800. I think he stopped selling because either people trying to steal his work or was tired of nitpickers, when they had done nothing themselves. Gary Sewall on the Aerodrome has his own plans, not for sale as I asked, but he said he would not mind contributing or reviewing if I came up with problems. I think his plans are possibly the best as he has been working on them for 40 years trying to perfect them; that's his hobby. I have seen snippits on the aerodrome.
I would be interested in trying to draw up a scale structure 1/4 Dr1 plan too.