Buying anything used isn't entirely dangerous regardless the the item. It does have it's ups & downs though. Using some prudent judgement, and doing some howmework/research on the seller and the intended item is a must however.
I bought what I thought was a 3 year old BAT VK 5i from a seller who I believe is no longer a member here. it turned out it was closer to ten years old in fact. The misrepresentation of it's age was of course, an issue. Once new tubes were installed in it though, I was very pleased with it's performance overall and held onto it for a couple of years until the upgrade bug bit me... and then I bought my Thor and sold the BAT.
Tubes in general last a pretty long time. Some say thousands of hours! 3000 and up isn't unheard of, some tubes life expectancy go upwards of 10,000, 'New' or NOS of course.
Used tubes can do pretty well too. it's all about what you want to do and can afford I guess. Getting some used tubes is an easy and cheaper way to find out what suits you too. here, again, the seller's reputation is most important. There are several here and off Audiogone which are most competent and trustworthy.
Age of the component is a factor too, though if the maker is still in business it is less an issue than if no original support is available, natuarlly.
My own preffs tell me to not buy vintage & antique gear, but some swear by it. Again, this is a matter more of preffs than actual betterment and remains quite subjective, IMO. I've lost the ability to properly serivice electronics so for myself, support is key.
I also limit myself to buying electronics that are but shortly used, if at all possible... 2-3yrs. max., and less if possible. The build quality of anything is gonna reflect in it's longevity and performance too. Again, by the time I sold my BAT preamp it was over ten years old and played flawlessly and sounded great!
Keeping in mind your situation, preffs, experience and so forth, it may be best to buy something inexpensive right off to see what is what in the tube world. Something with some popularity too, so reselling it if not to your liking won't be a big deal. Any of those companies I've listed above are worth investigating and you'll not go far wrong... if at all.
Sticking with a preamp whose tube array is current or plentiful is also a good idea. Like the 12AT, 12AX, 12AU, 6DJ8, 6922, 7308, 6sn7, etc. and they all have their own sonic signatures... which is good for you can buy & try out the same 6922 from any number of makers and customize the sound to your preffs.
Lastly more tubes inside the unit doesn't mean better all the time... it does mean more money to outfit it all the time though... but that'll only happen due to whims or every 4 or 5 years, depending on how much you play the thing. 4hr. per day = <1500 hours per year.
