Building a neck is far easier than you'd think, especially shaping it, which I was most concerned about and turned out to be rather simple and enjoyable.
The more challenging part is the fretwork, fingerboard and fret installation, leveling, dressing, etc., and basically all of the dimensional details.
I'd say jump in and do it.
As for your original question - is it ok to buy a warmoth neck? I don't think there are any rules about that
