Hard to say how long it will last but My FW900 lasted 8 years considering the build date of May 2002. I bought it used off Ebay in 2005 so I'm not sure how much use it had prior to that, but it was used heavily for the 5 years I owned it and had a great picture til the day it quit working.
The last FW900 was made in 2003 so at this point in time its risky business buying one used especially without a warranty.
Ive been considering buying the fully calibrated units on Ebay but they're pricey and the seller doesn't ship (local pickup only) which is about a 3 hour drive from here. (Link