Figure out what hubs you're going to use before you buy the rims - the spoke angle out of the rims is different for drum versus disk hub, but I think you already knew that, right?
For my tuppence worth, go with the Suzuki 4LS front brake - looks really period-correct even though it is blessedly heavy. A good machinist can get a few pounds off the assembly without sacrificing strength, though.
Check with Walridge Motors (Mike Partridge); he used to carry a spoke kit for the Suzuki 4LS brake / 18 inch rim combo, but don't know if he still lists it or not. I think he had 2 versions - one cad plated (which is what I used, it being a race bike and all) and one stainless for you guys with deep pockets.
Don't know what rear hub you're thinking about using, but the CB450/500 twins' rear hub is remarkably heavy, so maybe something else?
As far as the tank and seat, I believe replicas are available from a number of suppliers, so unless you have a tremendous urge to fab it yourself, might be an easier way to do it?
Have fun with your project. The Drixtons were a fabulous bike back in the day and there are a bunch of replicas being raced today.
Pat