I apologize, I saw your email and had every intention to get back to you, but I've had a hell of a week followed by a hell of a weekend. I slept until 3PM today and feel a lot better, so I'm trying to catch up. I typically recommend 9V supplies because there are a lot more of them out there and they can be had pretty cheap. Also since the high voltage is regulated internally so the tube will see the same high voltage regardless of the input voltage.
As such I don't have a specific 12V supply that I recommend, I would just recommend a bare minimum of 500mA, make sure it's good for a 2.1mm pin and make sure it's center-negative polarity.
With regards to your gain factor vs current draw question, there is a correlation between high gain factors and low plate draw, but typically the main instance you want high plate current is when you're using the tube stage to drive a load, which is not something I do much in my preamps. Because of how the circuits are set up there should not be much difference in current draw between different tube types. It should also be noted that the highest current draw in a preamp tube is going to be the heater, which is fairly consistent between all 12A*7 tube types. Plate current draw refers to the amount of current being pulled from the high voltage rail, and the difference in pull between 12AX7 and 12AU7 is only 28mA. This gets scaled a bit between the high voltage rail and the input voltage rail, but we don't need to get too far into the math there. Any of my preamps that were designed to use a 12AX7 (which is the majority of them) can use a 12AU7 without modification and will not be damaged as a result.