If you turn on error_reporting and set it to E_ALL then you should be able to see deprecated errors etc. I wouldn't bother targeting PHP5.2 and then 5.3.
Depends if you just want the apps to work or if you want to take advantage of new PHP features and performance improvements. If they just need to work then only fix the errors and ignore E_DEPRECATED and E_NOTICE warnings.
If the projects were written in a decent way to begin with then it should not be too hard to upgrade them.
That is not to say that it won't be a horribly boring and arduous job. It will also take far longer than you expect; particularly for 50 apps!
