I agree with Watty.
The brands you have shortlisted are all quality made products which do hold their value and with good reason. They are built well and built to last.
When we bought our KK it was nearly 5 years old, but in excellent condition. The whole thing had been well cared for and maintained. We asked millions of questions and got a good understanding from the owner about how and how often it had been used. His story matched the condition of the camper and we have been very happy with it.
Personally, I would do it all over again. A second hand hard floor that's 7-10years old, if looked after and used for the purpose they were built - should have many many more years in it.
Things to consider - Look for canvas quality and condition. As long as it has been put away dry and carefully, should be no troubles at all. Check the age and condition of any batteries and water filters in it. Also check out the mattress. If it's a foam one, you might want to consider replacing it as they do deteriorate with age. But all that goes for any brand of camper.
We replaced the gas struts on our boat loader, but the struts on the camper itself are/were perfectly fine.
You have to expect some general wear and tear of the trailer, but again, maintenance is key.
Of course I am biased, but I highly recommend the KKs. There are a couple of good second hand ones for sale at the moment with plenty of extras included.
Hope that helps.