This is a big and expensive model to be attempting if you haven't done a lot of magnetizing before.
The biggest issue is going to be "locking" pieces so that they hold a pose and don't
sag or rotate out of position. That probably means a combination of magnets and pinning or multiple magnets per joint, rather than sticking in the largest magnet that can be hidden within the model.
Michael Kan has a page about
assembling one
http://kan.org/michael/mkp/tyranid_hierodule.php, and he has magnetized other models, so it might be worth asking him for advice. I know he's on Dakka - just can't remember his user name.