I've got an 05 620, It was my first duc and I've invested a lot of time and love into this little bike, I reckon it will be something I will probably hang onto for a long time, a bike I'd like to pass on to the Grand kids one day ;D
However the fuel tank is plastic and reading about the problems that US bikes have had with expanding tanks that seems like it is related to ethanol in the fuel. In Australia we still have the option of buying fuel that is free of ethanol, so no problems with expanding tanks, but I can't see this lasting for too long into the future. Given that the steel tanks don't seem to be affected by expanding due to ethanol fuel, I wouldn't mind getting hold of a steel tank to keep on the shelf in the event that one day we are forced to use ethanol and my tank gets deformed.
But, do the old steel tanks fit the same as the newer plastic tanks??
Here's a bunch of info on Monster fuel tanks:
http://ducatimonsterforum.org/index.php?topic=541.0 (http://ducatimonsterforum.org/index.php?topic=541.0)
You'll need to get all of the pump, filter, and hose parts to go inside the steel tank, as none of that stuff from your plastic tank will fit.