Hy Lucy,
why you don't try  the ADS7825 by Burr Brown. It's a really powerful 16 bit
A/D converter with 8 bit interface.
It isn't very cheap, but it has got a 4 channels analog mux builted-in and
it not needs any external circuitry.
I'm using it in my LPTDAQ project.  I used the HI7810 by Harris Corp. too.
It is more cheaper than the ADS7825 (about 15$) but it has "only" 12 bits
and no mux.

Piovan Stefano
University of Padova - ITALY
Digital Control and Telerobotics
http://www.dei.unipd.it/~jan