ok so the MCP3004 is an SPI-based 10-bit ADC 4 channel, $1.50 in 5k volume, 200k-samples/sec, that's possibly waay more than you need? it's popular on adafruit and other places
and if you use the next one up from the PCA9536 - the 16-bit one - i think you're covered?
http://uk.farnell.com/texas-instruments/ads7949srter/ic-adc-8bit-srl-2msps-1...
that one looks ok....
l.