Elektron Musical Instruments

yeah! i've finally got the cash to buy a MachineDrum! I'm 
considering buying a used MD here in the United States to accompany 
my Nord Modular and am naturally feeling very cautious about it 
being a large purchase for a *usually* broke college student...

I believe i've seen a couple of posts on here about some nasty 
problems w/ the OS and some hardware issues and the like - is there 
a certain lot of serial numbers that I should avoid or anything like 
that? Is there anything else that you, as a more experienced MD 
user, would suggest that I look out for? I'd appreciate any advice.

thanks!

- Brian