The quality of the vessel is paramount. Often you’ll see wood or oak tanks and barrels on the marketplace that might seem too good to be true. There’s a reason for that. There’s a reason they are no longer being used by the original source, be it a winery, brewery, or distillery. One of the biggest red flags is the price: If it seems too good to be true, it probably is.
If you can, find a reputable source for oak and make sure they supply high-quality vessels. Knowing about cooperage, how it’s made, how it’s handled, and how it’s used can help you get a high-quality vessel. And knowing prior history is an important part of that, too. You can have a visually beautiful barrel that can give your beer poor or off characters.
Other things to take into account are the look and even the smell of the vessel. If it’s not well-made or maintained over time, it will not make good beer for you. So, do sensory analysis. Does it smell good? If not, it’s likely best not to purchase it. See if there are signs of the barrel leaking in the past; it’s not to say that you can’t repair that barrel, but it’s good to see these things in advance.