Has the car been run on e10 fuel?
The FSIs are not compatible and must be run on e5.
Though that being said, I don’t know if it could deteriorate the injector seals. I doubt it affects the rubber injector O-rings, (as the 1.4MPIs are compatible with e10, and those have o rings exposed to fuel with no issues) but possibly the ethanol could deteriorate the PTFE injector seals?
With regards to the manifold job, yeah it’s a bit of a pain to do, especially the first time, but not inherently hard. No Idea what garages would think of the task though as I did mine myself.
But as
@PlasticMac has said, the first step is definitely a good scan, otherwise you’re just throwing parts and labour at it on a whim.