I think downtheroad said it all.
I may add, the older the trailer is, the less it will cost, the closer to resale you'll get back. But, there is a trade off that downtheroad already mentined ... your tow vehicle. Older campers (10 years or more, that would return a closer to resale amount) were manufactured much heavier than today's versions. So the trailer weight is a big factor when you get older. Another thing, older trailers may have hidden problems that won't unearth themselves until after you start using it. Repairs (even short term repairs) could end up costing more than expected, and, if on the road, could cause a considerable inconvenience.
On the other hand, a brand new one could have warranty issues too, still taking it out of service when on the road.
But ... if looking at it from strictly a resale value ... used .. and older will yield a closer resale value. Buy new or nearly new, and it's a loosing proposition.
If you have an adequate tow vehicle, then the weight will be no issue. Considering your needs, I'd look at campers around 10 years old and go BACK from there.
EDIT: And when you resell your camper.... regardless of age ... you will never get 100% back out of it, what you put into it. You always put more into it ... always ...