I have done this in the past and used this program
Acarsd
ACARS - Free ACARS decoder for Linux & Windows - acarsd
KNOWN ACARS FREQUENCIES
The following frequencies are used to transmit ACARS data to and from the aircraft. All frequencies in Mhz.
FREQUENCY REGION / COUNTRY
131.550 Primary Channel worldwide
129.125 Additional channel for USA & Canada
130.025 Secondary channel for USA and Canada
130.425 Additional channel for USA
130.450 Additional channel for USA & Canada
131.125 Additional channel for USA
136.700 Additional channel for USA
136.750 Additional channel for USA
136.800 Additional channel for USA
136.850 SITA North American Frequency
If you need help setting up acarsd let me know and i will help you. You don't need a scanner with a mod tap just a good sound card and a stereo or mono cable from the scanner headphone jack to the line in of your PC. You could use any basic scanner with the air bands. Try it first then decided if you want a dedicated unit just for acars.
Just my 2 cents
Mike