Did the person you're looking to buy it from have it rebuilt themselves? If so get all the details you can, as others have said above it looks to be a rebuilt CT26/"20b". If the seller doesn't have any info on it (came with the car etc) see what else has been done to the engine and if they have a dyno chart so you can see what it performs like, to give you an idea if it's a standard rebuild or a hiflow.
Another option would be to phone GCG with the info stamped on the turbo and see if they can give you more info on what was done to it (and when).
If it's been hiflowed it will likely have upgraded internals making it a little more laggy than a stock turbo in most cases, but able to produce and hold more boost at higher revs; the last pic you posted looks similar to the steel wheeled ST205 CT20b wheel.
CT20b's aren't amazing turbos by today's standards, plus they are all going to be at the very least 13 years old so don't expect crazy drastic improvements over a CT26 or finding one without shaft play that hasn't been rebuilt, but in saying that they are better than a CT26, just some food for thought.