New or used doesn't really matter where it's not that used (obvious signs of wear on gear teeth). If the Tarmac deal falls through, you can get a lot of bike by staying away from the big names and considering a Jamis Ventura Race or KHS Flite 500. Both are ~$1000 new and come with decent 105/FSA drivetrain components and might come with a bike store fitting, too. It's not just a matter of "medium" or "54 cm" ... it either fits or it doesn't, regardless of what size label it has been given by the manufacturer.