Like Mike said, read the button, store the value, output that value as a voltage level for >1 minute, and then zero.
I'm going to recommend a PICAXE microcontroller.
1. Originally intended for Elementary and High School classes (think easy)
2. Free software and manuals
3. Programming hardware is 2 resistors, 5V power (batteries or wall wart), and a three-wire serial cable
4. Programming language is BASIC
5. Very helpful and civil Forums
6. Variety of chips from 8-pin (~$3.50) to 40pin (~$10)
Down side is they are slower than PICs with compiled programs (not important to you), and more expensive (if you don't count the PIC's programmer).
The PICAXE 14M would do the job. 5 inputs, for your pushbuttons. 6 outputs. Use one output for a PWM digital-to-analog conversion, if you went that route. Or, use 5 outputs, if you want to go with an R-2R ladder digital-to-analog conversion (my preference...but that's just me
).
PICAXE 14M: **broken link removed**
R-2R network:
RGSD10L503G Murata Resistor Networks & Arrays
Ken