AvePoint is good tool for migration, but It will not meet all your need for
migration. I am not sure if they can migrate any of your custom developed
components.
In Sharepoint 2003 there were alot of area while in MOSS there are no area any
more. This is something you can look into ave point.
In brief, AvePoint will smother the process but you will still need to do alot
of work on your own.
I will say a better planing and mapping should be done ahead of any migration.