Hi Neil
It sounds as if you could have an exciting project in front of you, if you take it.
You could be at the bottom of a steep learning curve, or the edge of a mine field. Get it right and you’ll have a great boat and great fun at surprisingly small money. Get it wrong and it could be an expensive mediocre experience.
The junk rig can be a great rig, easy to use and giving great performance, and the Halcyon 27 would be a good boat to put under the rig. There are a lot of older designed rigs about that don’t perform, and you will want to steer well clear of them. It is because of them that you have already found that there is a lot of miss-information about the rig and been told they don’t work on long keels.
Look at Arne’s rig information and you won’t go far wrong, though he likes generous sail area for his sailing ground. You have a lot to learn, but it is well worth the effort. My advice would be to rush slowly, but a junk rigged Halcyon 27 would be a great boat.
Good luck.
Cheers, Slieve