Unless you know what specifically is wrong with a Kirby, DO NOT buy one from a place you could expect some problems.
An abused Kirby can get pricy to fix. The G5 I gave to a family member needed an armature, carbon brush, mini-emptor, and fan replacement after they abused it for 8 years. It got really pricy, really quickly.
However, if it powers on, sounds healthy, and doesn't have any physically broken parts, then you should go ahead and buy it. My Sentria was $40 and all it needed was a new outer bag and fan. I only bought it because it ran well and sounded healthy.
Like I said, I do not like carboot sales for Kirbys, but you can definitely go and buy one there. Including if it's under 20 euros and in decent condition. Kirbys can almost always be fixed. I pulled a G3 out of a pond and got it to work, but it was expensive. Just trust your gut, you know what a healthy vacuum sounds like, and if you don't, YouTube "Kirby vacuum" and you'll be able to hear a good amount of healthy Kirbys.
Oh, and to answer your question earlier, I have a DC07 Animal. I wish I could get rid of it. It was $15 and definitely not worth it after I've replaced several broken parts that happened just from using it to clean my carpet.