Personally I would rule out the easy and cheap stuff first before finally saying it's the turbo. Although turbos can fail at early mileage, for it to fail at 55k miles is rather odd. Secondly, you say that the rattle only happens below or on 30 mph, but at stationary and any other speed the sound goes away - I reckon it might be a worn suspension component. Things like, CV joints and ARB bushes can fail at that kind of mileage; they do need replacing often, especially in weather-beaten, pot-hole ridden roads. If they're worn then most certainly you will get rattling and clonking sounds if you drive over rough road or pot holes, etc.
Reconditioned turbos shouldn't be more than £300. Anything more, then in my opinion, you're getting ripped off. You can buy new turbos for around £500-600. But fitting costs will depend on your mechanic. Just buy the same turbo as your old one, you don't need a different one unless your uncle is after a turbo-charged stallion (o..k maybe not for a Sharan!)
Innotec is just a turbo cleaning solution - basically cleaning the turbo's guts out to get it squeakly clean. Often helps to resolve limp mode problems, especially if the turbo is clogged up with carbon deposits, etc. Obviously, not important in your case if it turns out the the turbo is flubbered, but for the majority of us, it is simply a service cleaning solution for the turbo.
