I'm not an expert at this, but I'd think spending as little money as possible for a winter bike would be best. But then again... I'm cheap
One of my co-workers went the Wal-Mart route for a winter bike. Don't know how much he spent, but it couldn't have been that much.
I think I've also heard of others buying used bikes every other year or so for winter use.
Either way, winter tires and fenders are probably the best things to add on to any bike.