I have just uploaded a link to the Dropbox folder containing all the design files so far (no PCB yet). The schematic is still unfinished, but it's getting there, and the online file will update as soon as I save a new version of the original.

I have decided to use an analogue switch for attenuation selection because they are cheaper, have a very high -3dB bandwidth and have a minimal capacitance between contacts. I chose the ADG734 which is a quad SPDT switch: 2.5ohm on resistance and 160MHz bandwidth. The larger on-resistance will slightly affect accuracy but not much. I may make some of the smaller attenuation resistors 3 ohms lower or so to try and negate the effect. Since there are 8 attenuation bits, there will be two ADG734s, probably controlled by a 74HC595 shift register rather than another PCF8574 (which is used for the LCD); they are one fifth the price.

I am also following rob77's suggestion on this thread to wire the keypad up so that only one ADC pin is required to read it. This will save me another PCF8574, so the only one being used now is on the LCD.

These changes should appear automatically in the Dropbox folder linked on the project page.