FlightCheck (watch movie here on how http://uk.youtube.com/watch?v=r8uCud5-UhM )works excellent for preflighting PDF's. Top-notch really. There will be a new version of FlightCheck Professional v6 that will be coming out in a few weeks that will also have a built-in online preflighting option.

This will be the route to go, as it is very economical, yet extremely powerful- allowing you to offer from your web pages full online preflighting, job collection (all fonts and images) and FTP file transfer, right to your FTP site or a managed service online.

Contact Excell Technologies http://www.markzware.com/distributors/europe/south_africa/ in RSA for more details, coming soon!
(If you would like to beta test, drop me an email on david AT markzware DOT nl)