Spash,
Problems with blown head gaskets on postwar Douglases seem to be more common now we are forced to use non-asbestos gaskets. Having said that, there are also a couple of other factors that may contribute to this problem. Firstly, the head studs would benefit from a couple more threads to prevent the nuts going down thread bound on the studs. Also, there is not much room for the spanner - on my Dragonfly, I have used 5/16" nuts opened out to 3/8" so that I can get a ring spanner on the smaller hexagon. Some owners have also found that head gaskets blow after having the heads and barrels powder coated - I have put this down to the coating squeezing out from under the nuts when it gets hot, thus easing the clamping pressure. Regular checking is a must!
If you do decide to buy the bike, replacement of the gasket is a straight forward task that shouldn't cause the average owner too many problems.
Also, if we knew your location, there may be a fellow owner in the vicinity who could give some practical help!
Regards,
Eddie.