Re: seven segment
I am not sure how are you going to address letters such as M and T with 7-segmanet display - maybe you should consider, say, 3 LED per segmant and build the display of discrete LEDs ..
Anyhow, I wouldn't use 7447 decoders at all ..
Instead, I would connect Anodes of segmants (or diodes) that you need to illuminate through current limiting resistors to + supply and Cathodes to one NPN power transistor ..
This transistor can be driven by a small microcontroller and to achieve the dimming effect you can implement PWM (pulse width modulation) procedure ..
Regards,
IanP