is it possible to take a turbo from a 900SE and put it in a 900S?
It should be. As long as they have the same kind of engine (most likely a 2.3L I-4), which they ought to, the turbocharger should be interchangeable. The 900 SE was just a higher trim level, i.e., wooden dashboard inserts, leather, etc.
go to saabcentral forums and ask around, im not sure if they used the same internal components in both models.
Search Saab 900 Questions
Are you a UK consumer? CarGurus now has a discussion forum in the UK.
Saab 900 Experts