magnetise the weapons on the larger beasts. (warriors upwards).
WYSIWYG wise, weapons should be on the model. Stuff like adrenal glands could be internal, and things like scuttlers don't have to be modelled. However, your opponent should be left in no doubt as to what everything is equipped with, so clearly state if anything isn't shown on the model.
As an example, I've played against an army that had 3 warriors armed with deathspitters and scything talons, no biomorphs on the model. during a critical part of the game it turns out they in fact had talons, rending claws, leaping and associated combat biomorphs. A unit of mine I thought safe was well in charge range and got wiped out, allowing him to follow up into another unit. For some reason, my opponent thought it was okay to simply write it down on his list and not actually bother to tell me.
I'd assemble big stuff with magnetic weapons now, but leave off biomorphs. These can be stuck on later. Gaunts are more tricky. We won't find out which weapon options are best (if the writers done his job correctly, then none of them will be 'best') or which suit your play style until the codex is released. If you are easy going with your army then just use the ones you like the look of. If your a hard core tournie player, wait for the book.
One thing though: grab a couple of the current battleforce boxes. They are being delisted and will no doubt be replaced with contents that are not as good a deal. Currently, the carnifex is almost free.