It's a business management application ... So it really kinda has to be a MDI so users don't have to contend with lose windows scattered all over the desktop while looking up multiple clients/products/contracts/etc.
-Stoic Joker
I've done something like this in SDI with WPF, and it actually turned out better once I got rid of my preconceptions.
Two things I did to make window management simple:
1. The windows are aware of the other windows and there is a menu that allows you to select the other windows from that window
2. There is a launch application that everything is launched from, which also has an icon in the tray notification area.
How this helped and became a help rather than a hindrance:
1. They each run in their own process
2. All of the client machines have two monitors, so the MDI interface that they were used to actually limited their ability to position windows; they used to run two instances of the application.
3. They all are upgrading to windows 7; the positioning abilities of windows 7 actually made the layout more versatile.
So I'd say don't limit yourself by the classic UI, but look at the options and find out what actually works best for the client.