Nothing wrong with purchasing used, just inspect it thoroughly before putting any money down. Make sure everything works, no leaks, try it with slides in and slides out, if it has slides, make sure everything is accessible (new or used for that matter), check your towing weights, and if purchasing used, ask for any documented history, like service calls, repairs, things like that.
If purchasing new, you no doubt will have some kinks to work out. If you have a few months you can use the camper before going full time, you can work through warrany issues, make some Do-it-yourself modifications and changes, (which usually take some time to figure out) (I just made a cabinet change yesterday after owning mine for over 3 years now, sometimes it just takes time to figure out what needs to be changed or modified with new). With used, the previous owner may have made some alterations. They may work for you, they may not, so check the used unit out thoroughly, not only for structural soundness, but also for actual comfort and usability.
If a used unit is in good working order, the floor plan works for you, and there are minimal modifications you see in the future, then going used will save you a lot of money, money you can use for camping and your future retirement.
It's really your decision. But which every way you go, new or used, check out the functionality with slides in and slides out. You really need to have access with slides in.