Hi,
Second hand GCs are like second hand cars, projected life depends on the previous owner. You can tell the manufacturing date, and go for the cleanest and youngest you can afford of the desired configuration.
I've worked with 5890s since they first came out, and if they are well maintained they will last decades. The main failure I've encountered is the oven motor bearings and the glow plug ignition for FID - both cheap to fix. If they have been in a lab with good electricity supply and good clean dry gas supplies, that will minimise future issues.
My previous employer has two 5890As and a 5890A Series II still going strong - just retired a 5792A ( the 5890a predecessor and one of the best GCs ever... ) after about 20 years continuous use, all with autosamplers.
I haven't had much experience with the later versions with electronic features, so can't comment, but would caution about buying old GCs if you want to hook them up to modern data systems. It's almost better to buy the whole deal of the era ( eg 5890a Series II, Chemstation on windows NT or whatever ).
Note that many of the 5890II originally worked with hardwired integrators ( eg 3395s ), but Windows PC Chemstation systems would be preferable, ideally with HPIB rather than HPIL, and a later release of Chemstation ( after A v8.xx ). Check around, as some companies will shift them because Agilent suggort has ceased, and many will hardly be worn in. Autosamplers are nice, and should also be preferred.
The issue of spares shouldn't be too traumatic ( given the 100,000 or so lurking on the planet ), but if these GCs are critical to your business, then push you boss to buy new, even if only one now and the other later. Buying second hand GCs for critical applications is likely to invoke Murphy's law.
I hope this is helpful,
Bruce Hamilton