Look into the insurance requirements as well. Aerostar's will have some fairliy specific minimum experience level requirements.
Navajo is a great plane, but I would not recommend it as an initial. The Aztec is a unique airplane. It may be slower, but it will carry a full load of passengers and bags with a pretty decent load of fuel and be legal. More importantly, it's the only one that will CLIMB on one engine with that load.
The cabin is not a limo, but comfortable and its a very stable IFR platform. From a training standpoint, it can't be beat. Yes, it will work you because it feels like a bigger airplane. You will learn to think ahead, fly and follow procedures. It will also leave you with a solid level of confidence and the ability to transition much easier to any other twin out there. Having flown the 400 series Cessnas and Navajos as well, personally I think the Aztec is better suited to single pilot IFR than they are, strictly from a layout point of view. Everything is a bit closer and easier to monitor.
That last comment was not intended to start a firestorm, just personal experience from having flown them as single pilot and two man. You can do it a lot easier with a coupled AP, but when you need it most is when it's going to decide to take a vacation.