For a) something like trend serverprotect for linux could be used (you'd need to store attachments as files) but at $500 (USD) its not cheap.
http://www.trendmicro.com/products/splinux/ . Mcafee offer a similar product called
NetShield for NetWare or NT, $400 (USD) or for the hacker out there "
Symantec AntiVirus Command Line Scanner" NT/Solaris/Linux/NetWare could be integrated $40.
For b) I'm not aware of any useful online virus scanner which could be used in conjunction with vB all online scanners I've seen are based on the premise of running the scanner as a local ActiveX control which is not useful for doing automated scans.