All of the 1/6 guys on those sites are way bigger than a Slim Obitsu and then it depends on the type of neck you need. Take some pictures of the head, including the neck hole, and I can help you narrow down what to look for. Hot Toys and SIdeshow are huge compared to Slim and the head might look too small.
I actually don't have a head yet either, and was planning to order that when I picked a body. I've noticed, tho, that most of the action doll types have what I think of as a 'ken doll' neck, where the head can only turn from side to side and has limited mobility... I don't really like that, as I think it loses a lot of expression/body language potential that's needed when you're dealing with an actor/model that can't actually change face expressions.
I'm also still debating the magnet feet issue when it comes to the obitsu. On one hand, I want as much posability as I can get. On the other, I've heard the magnets are kind of weak and won't work well when the doll's actually wearing shoes... So I'm wondering if I should save a few bucks and just make shoes with magnets in the soles?
The accessories really ARE to die for, though
I know. If I get nothing else from that site, I will probably save up to do an 'accessory raid' order at some point in the future!