Probably the SY22. I’ve never built one, but many have with success. The NULL is supposed to be an improvement over the NAG, but I have yet to try one of those either.

It Is known to be an unfinished model. Although it contains some very intuitive design features, there are too many flaws to be worked out as it is.

I’m not saying that it’s impossible to get one working, but it’s not going to be a fun time, and will require overhauling many of the models.