Your datarate requirements sound doable on an Arduino. This is not really
very high-speed. I don't think it was mentioned, but you can pack the 8 digital
signals into a single byte for compression during transmission. Then you only
need to send 3-bytes with each transfer, 1 for digital, 2 for analog [10-bit data].
You need baudrate = 38,400 or faster.
I'm not sure it was explained completely, but if the ground on the system you're
monitoring is safe to connect the Arduino gnd to, eg, not a floating or AC circuit,
then you can feed in the 24V signals simply using 5:1 voltage dividers. Actually I've
seen appnotes [Microchip, not Atmel] where they connected to 120VAC signals,
but you need to be extra careful in picking the proper ground wire, or you'll blow
the system.
Also, Arduino will only convert positive-going analog signals, so you need to
translate the +/-10V using an opamp or voltage divider with pullup voltage.
If you don't need to read the actual DC level of the analog signal, you can
capacitively couple it.