As indicated earlier - a new bulb is a must. Even if your source says it's new, it would be nearly impossible for you to determine if it is still generating the necessary power in the correct wavelengths to have a suffecient "killrate" for micro-organisms (unless you had a considerable amount of light analysis equiment including a very sophisticated digital spectrometer). Their typical effectivity is a year.
So, as mentioned by gfishguy and fishfreek, - you have to buy a bulb. That being the case consider the following example: a 25 watt Rainbow sells for about $150, the bulb sells for about $50. Therefore if you were to buy this unit used for $90 and then bought the bulb new for $50 your total cost would be $140. Thereby only saving 10 bucks and ending up with an old UV
unit and no warranty or support.
Also be careful about one other thing here - the sleeves that the bulbs fit into are of critical importance to the effective transmission of UV
to the passing water. Many of these are Quartz based. Over time the UV
alters the property of the quartz sleeve and it has to be replaced as well. Manufactures differ on the life span of this sleeve but needless to say if the unit is very old this may require replacing as well (for a 25watt unit these can run about $50 as well). Even if the sleeve is not quartz - its clarity likely suffers in some environments (i.e. carbonate deposits etc.) so this is something to consider,
Bottom line: I think it's better to buy new unless you are sure of your source - the type of unit, how long it's been in use, etc.
Also - recall that the "kill rate" of these devices is directly related to flow rate through them, i.e. the dwell rate of water inside the unit. Be sure you consider this in your choice.
Hey hope this helps.