Hi,
You can use the ADC0808, which is 8 channal ADC with the conversion speed of 100ms and give only one o/p at a time but it wont be matter because its conversion rate is quite high so that you can seelect the channal and read the respected ADC input on the single port and can do the appropriate action as per your system requirement. It is not practical to use all the 7 I/P simultenious, you should have to select one by one. And this is the cheapest ADC with this much conversion rate.....

All the best
Sandeep...