I was just in MEX for the first time also and had the exact same concerns. Think I posted my detailed arrival and transport under the Intercontinental Mexico City forum. You should have no issues purchasing your voucher upon arrival and taxi the airport sanctioned taxi to your hotel. That time of the morning should be a breeze in terms of traffic. Do not waste alot of energy worrying about this. I already did for both of us. I found Mexico City full of culture and friendly people. If necessary would return without any hesitation. I always get everything lined up properly like you. Enjoy!