If money is no object, buying new gets you that bright shiny experience and some warranty coverage. Money is somewhat of an object for most of us so buying used saves a lot, plus someone else has wring out the initial bugs all trailers have.
We have purchased 2 new (TT and pop-up) and 2 used (TT and 5er). The last two have been the used ones and we've been happiest with them. Purchasing used allowed us to buy higher quality units AND save money. You need some experience, or someone that has some to make sure you get a used one in good shape. Be wary of dealers, although not all are bad.
You'll find it's better to have an overmatched TV, especially if you're not used to hauling a big trailer. I still remember the white knuckle experience the first time I pulled, fortunately this goes away quickly with experience...even quicker if your TV is over capable. I love pulling now, more than driving without a tail behind me.
Read this forum a lot, need to filter some of the advice but you'll learn a lot!
Interior layout preference will develop as you gain more experience, hence the advice to be prepared to buy a couple units over the first few years until you know what you really want.
Like most that try it, I suspect you'll love it and be sucked into the vortex...many buy a truck, then a trailer that's a little large for the TV resulting in a new truck, then a new trailer because TV can handle more, then a new truck, then...
Keen an open mind, you'll learn a lot and have a great time.
Good luck!
