>>140529
Depends what waters are you diving in, cold?
How thick is your wetsuit?
For a person with 70kg on a wetsuit of 5mm, 5kg is enough.
If arround 5-6kg I'd recommend using it all on your waist, preferably on a silicone or rubber belt.
If you need more weight then I'd use a vest like picrel. Forget the neckweight.

I strongly recommend joining a freediving training group (pool), you will develop the skills you need and you will learn alot, not to mention the great people you will probably meet.
I did it and now I'm addicted to freediving