Unvented are potentially very dangerous. Common sense if you ask me. You don't walk around all day with a plastic bag over your head, why make your house into a similar situation?
As for mantels, It really doesn't matter if you use old or new, just so long as you strictly follow the manufacturer's specifications. Any approved manufactured fireplace has specific distances to combustibles for the sides and the top. The odds are that your old unit won't fit, but you may luck out. You can spend under $500 for a really nice one here, including custom sizing (painted, not natural finish) here: Appalachian Mantel Company, especially if you check out their ongoing ebay.com auctions. http://www.netease.net/amc/ (I have no affiliation, I was just happy with them)