LOL - I LOVE things like this...
I run into them all the time at work. Companies and people that are too cheap to buy the NEW server hard drives and opt for the refurb ones. Then I get a call 6 months later that they want a refund because the drive failed. It's missions critical server, blah, blah, blah. I tell EVERY customer who buys refurb drives that it's 90 days and I wouldn't if it's a critical server. So when they call, I tell them, why didn't you spend $100 more and buy the new drives? They get sooo pissed. Certain items are MORE than fine refurbed. Basically anything that doesn't have moving parts. Memory, Processors, etc.
You are taking a chance buying used. Didn't want to fork up the money to go buy a new aftermarket one (I'm assuming they are available, I don't know e30 stuff). If you wanted 100% sure effectiveness, fork up the dollars for new.
2007 Z4 3.0si