Th 9401 150s are nice little boxes, I've got one at home running OS/400 v4r5. I use it now and then, but then programming/administrating the AS/400 & iSeries systems is my bread and butter. Anyway, it's only when you get to v5r1 that you need a licence for the OS for this particular model - licences are terribly expensive though, don't go there. Although saying that, you are unlikely to be able to get v5r1 now as it is defunct. End of life for OS/400 is currently as follows:
v5r1 - Sep 2005
v5r2 - Apr 2007
v5r3 - Apr 2009
v5r4 - to be announced
v6r1 - to be announced (this is the most current release)
The one your link points to doesn't appear to have an ethernet card, so that would be more expense. It probably doesn't come with install media, but you can (and should) create your own complete system backup onto tape that can be used to re-build the system.
Is it vintage? I wouldn't call it vintage at circa 1997 for this model.
Would it be worth to have? Depends on why you want/would like it. I wouldn't spend the money on it that they are asking in your link. I've seen several of this model go on ebay in the UK for anywhere between GBP 100 and GBP 500 (I was lucky, I got mine 5 years ago for the price of a pint of beer :D).