I wouldn't be concerned about buying it used if it can be verified to be in good working order. I bought a beautiful classic 3 for about $20. I polished it until it gleamed like new. Unfortunately it is well overdue for it again. I regularly see them at thrift stores around here of varying ages.
I've never paid over $20 for any of them, the newest being a G5. As far as bedbugs/bags, I'd be more concerned to check the fan blades in case someone got the wise idea to do something not so smart.
My single biggest gripe with Kirby has always been how heavy it is and how much extra work is required to use attachments. I've always preferred canisters to uprights for attachment ease. I generally use the uprights for quick carpet jobs but canisters for the whole deal.
I've always had a bias towards a good bagless, my wife enjoys the Rainbow. What I like about the Rainbow is a lot of its tools are cool and genuinely useful and easy to use. They actually have done a lot to improve them, compare a 1996 D4 to the current model E2b. The MiniJet, RainJet, Aquamate and Supermop are all extremely easy to use and quite effective, they extract well as carpet cleaners due to the motor power. If I want to comprehensively do a lot of stuff the Rainbow, or most any canister with a lot of tools is a lot more useful.
I prefer the Kirby when I want a straight carpet cleaning. Of all of mine I prefer the Classic 3 and rather push it myself than the self propelled models. A key thing to be advised is make sure the height is adjusted properly. A lot of people don't. On Rainbow demos I have had with Kirby owners a lot either have it digging in too much or too high so it skips across the carpet.
A point to note. The Kirby cleans well... If you have the height adjusted properly. I did a demo recently where the people had two Kirby's and it was the worst I've been on. They put a new bag on theirs before they swept because they thought that the airflow was the problem, the vac went thump thump thump as they used it, they had the height too high. Had nearly 1/2 inch of thick mud when they vacuumed a 10x12 small sitting room. It was quite disgusting, it was beyond just a bit it looks worse in water. It was also very avoidable. I'm not sure who sold them the Kirby but that was a lot of dirt and would have made even a $40 vac shine, if they were slightly lower on their height setting that would have been much less of an embarrassment to them. It wasn't even my fault, they simply were never really told (I informed them).
The reason I'm telling you that is because the cleaning difference if the height is wrong is quite substantial, I'm free to mess around with these vacs all the time since I own them (and have to practice Rainbow demos, since I back my wife up on it being the only vac that doesn't trigger her asthma).
If you ever decide you want something that you actually want to use all those extra tools on, the Rainbow is the simplest I've used for diversity of its tools, but you don't have to pay for the newest model if you can't afford it. Everything from the single speed E Series on up interchanges as far as attachments but you need the newest hose for water pickup (unless you have the previous liquid hose) and the most recent powerhead needs the the wand portion. Technically you can make anything fit if you really want to even to a D4 if you mess around, wiring harnesses are abound. The D4 is one heck of a workhorse, can't tell you how many I called from 1986 just today to see if they needed service that were going strong after 28 years, it makes me reluctant to get rid of mine ever.
I can tell you used is the way to go, by all means. I can tell you I bought 3 Kirby's, 2 Rainbows and 2 Dysons for less than 1/3 you could talk a Kirby dealer down to on a good day, and the two Dysons were new and my E Series was the current model when I got it.
