Lester Grau has written, or assisted in writing, two very good books on the subject (from both sides). They don't focus purely on the equipment involved but rather the battles that took place.
Nevertheless, they are free and may be worth a read if you are interested in the subject:
The Bear Went Over The Mountain (Soviet side)
http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA316729Other Side Of The Mountain (Muj side)
http://www.tribalanalysiscenter.com/TAUDOC/Other%20Side%20of%20Mountain.pdfIn addition to what others have suggested, you could add:
2S3 Akatsiya - Trumpeter recently released this.
2S1 Gvozdika - Trumpeter has this coming too. I'm not sure if they used the 1 or 3 in Afghanistan but both is a possibility..
T-55 MTP-3 / BMR (mine clearing variant) - Not sure if any aftermarket companies have this available but it's possible.
TOS-1 - Technically it wasn't "used" but it was tested in the Panjshir Valley during the Afghan war. I think there is a 1/35 model out there somewhere for the TOS-1.