A variable power supply (PS) isn't the answer for digital unless you just want a PS with higher amperage for your track and you set the voltage to the required number and leave it there. (Beware: Set type power bases aren't robust enough to handle too much higher amperage than stock. If you use stock motors, you shouldn't have a problem but a properly sized fuse in the system is a must) I don't have Carrera Digital but I have SSD. My power base allows me to adjust the power level going to each of the six cars that can be run on the track. From what I hear Drewbert34 saying, it sounds like Carrera has a similar function. My advice, the stock Carrera PS is probably good enough for your use. If you get into some serious racing with a track running a full group of cars, you can look into power supplies later.
That brings us back to what is the minimum necessary to chip a Scaley Trans Am car to run without magnets on a Carrera system. They are:
1) figure out a way to install the chip. It will most likely involve cutting and raising floor of the full interior of the car to make room.
2) remove the magnet. Add weight as necessary.
3) switch to aftermarket rear tires, either silicon or urethane, your preference.
4) Optional (This falls under no mag tuning. It's a disease most slot car racers eventually get)--------- If you experience excessive wheel hop coming out of corners, do the super glue trick on the bushings to take out excess axle slop. Axle slop is due to Scalextric's use of splined axles. The same goes for running Carrera cars without magnets.
Have fun. My Scaley Trans Am cafrs are among my favorites.