I'd try to get the most realistic neck possible (not a Barbie neck) at your local beauty supply shop or wig shop. They might sell you a used one that could save you some money. I had one for years that I picked up at a yard sale. Sorry but I gave it away years ago after my flash experiments were done. They used to sell a beauty toy for young girls that had a mannequin head in the set. Good luck
Whatever you get, get a mannequin that faces forward and has a straight pose. I have a "posed" bust of a lady looking over her shoulder, chin bit raised, which is good for only one lighting scheme. Don't limit yourself. I got mine on a whim, at an antique store that doesn't take returns, and the uselessness of it only hit me on my way home. It's been sitting in my garage ever since. Think first, then think again, and then whip out your card and pay.
It seems that every month is so, some department store closes its store in town and has a final sale. An acquaintance of mine makes custom costumes for period actors and has bought several mannequins from stores that were about to close (or, by contacting the headquarters, from stores that have already closed and are awaiting renovation).